The downward trend in offense was easily understandable. And that’s what makes the 2015 season so mysterious.

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Last season, MLB’s home run total jumped by 723, or 17.3 percent, a spike not seen since 1996 when homers boomed by 21.6 percent in the nascent days of MLB’s so-called steroid era.

Since 1970, baseball has only witnessed six seasons in which the home run total increased by 700 or more. Five of them are explainable by events in and around the game.

The first such spike in 1977 coincided with MLB’s switch from balls produced by Spalding to a new version made by Rawlings, the same manufacturer used today. Hitters launched 1,409 more home runs that season. In 1982, the second 700-plus home run spike, baseball was returning from a players strike that cost the league 38 percent of its season in 1981. The 1994 strike produced a similar result, with a 775-homer increase in 1995. Those seasons were bookended by home run explosions in 1993 (992) and 1996 (881) that correlate with the start of widespread PED use in the game.

Then there is 2015, when baseball’s offensive downturn was reversed in remarkable fashion. The odds of such a development are so slim, they were almost incalculable to Robert Vanderbei, a math professor at Princeton University.

Vanderbei used MATLAB, a high-end program for numerical computation, to determine the odds of such a spike following the offensive droughts of 2013 and 2014. The result?

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“It said zero,” he said. “Something definitely changed. I don’t know what, but something definitely, significantly changed.”

But what? What can explain such a sudden and violent reversal of baseball’s recent trend? As players prepared for the 2016 season in spring training camps across Arizona and Florida, no one had a solid theory to explain the explosion.

“700? That’s a lot,” Kansas City Royals starting pitcher Chris Young said when presented with the figure. The veteran of 11 big league seasons couldn’t think of a clear reason for the spike.

Washington Nationals ace Max Scherzer similarly lacked an immediate answer. He wondered how many ballparks had moved in fences. Then he wondered if the ball had changed, noting he would not necessarily be able to feel all alterations while throwing it.

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“Every year parks bring their fences in, don’t they?” said Shawn Kelley, who pitched last year with the San Diego Padres.

The Padres, along with the New York Mets, did move in their outfield walls last season at their home, Petco Park. “It used to be the hardest one to hit in and last year it wasn’t,” Kelley said. “From what I heard going there, to what I saw when I got there, there were a lot of balls that went out where I was like ‘I wouldn’t think that would be a homer.’”

Changes to stadium dimensions did play a role, but the alterations in San Diego and Queens only upped the totals for the Padres’ and Mets’ home parks by a combined 99 home runs from 2014 to last season.

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Other theories included climate change, improved scouting, even a conspiratorial suggestion the ball itself may be livelier. But a closer examination yields no single, clear cause to explain one of the biggest offensive outbursts in baseball history.

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What it wasn’t

Some offered explanations can be eliminated, however. It’s true 2015 was the warmest year on record, and it’s also true baseballs fly farther in hotter temperatures. Alan Nathan, a professor emeritus of physics at the University of Illinois at Urbana-Champaign, found an increase of average temperature by one degree Fahrenheit would result in an increase in home runs by about 0.6 percent.

Per baseball-reference.com, average game temperatures during night games increased from 73.0 to 73.7 degrees from 2014 to 2015, and from 71.6 to 73.5 during day games. But to fully account for the 17.3 percent spike we observed last season, temperatures would need to jump from those averages around 73 degrees all the way to 361 degrees Fahrenheit across all games. Thankfully for mankind, that was not the case.

Equipment changes can also be ruled out. While Barry Bonds may have popularized using maple bats, over the then-more common ash, when he set the single-season home run record in 2001, the lumber is a non-factor.

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“If a baseball player thinks they can hit better with a maple bat, maybe they can hit better,” explained James Sherwood, director of the University of Massachusetts-Lowell Baseball Research Center. “But from a scientific perspective, there is no difference.”

One MLB scout with over 30 years’ experience suggested the possibility of a livelier ball, the theory also put forward by Scherzer.

“There are smaller seams on the ball so it is harder to make the ball move and break on breaking balls,” the scout said. “I also think the ball is wound tighter than it has ever been.”

Changes to the ball could explain the jump. In 1974, when Spalding was still the manufacturer on record, the ball’s outer coating went from horsehide to cowhide, and home run production dropped nearly 15 percent from the year before. Rawlings, the current manufacturer of MLB baseballs, took over from Spalding in 1977, and home runs jumped 63 percent in a single season.

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Changes — no matter how slight — can have immediate effects on the league’s home run rate. But not in this case, according to Rawlings.

“Major League Baseball audits our plant at least once a year and is completely satisfied with the ball we produce,” said Mike Thompson, executive vice president of marketing for Rawlings. “There is ongoing testing all year long just to make sure the consistency meets Major League Baseball’s needs. If anything, the tolerance and specifications on the ball have been improved. And the rigidity of those tolerances has been tightened.”

In 2000, Major League Baseball and Rawlings funded a study by Sherwood at the University of Massachusetts-Lowell Baseball Research Center to study the baseballs being used. Sherwood, who continues to examine baseballs for MLB today, found “no significant performance differences and verified that the baseballs used in Major League games meet performance specifications.”

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So, was it the players?

One possible explanation stems from the significant influx of young talent into the game. Last season baseball debuted arguably its best-ever rookie class, one that included Chicago Cubs sluggers Kris Bryant (26 home runs) and Kyle Schwarber (16), Los Angeles Dodgers outfielder Joc Pederson (26), Houston Astros shortstop Carlos Correa (22) and Twins infielder Miguel Sano (18).

In total, last season’s rookies blasted 714 home runs, an MLB record. But that total still doesn’t account for the entirety of the spike. There are rookies every season. When you adjust for the home runs hit by rookies in 2014, even this record total accounts for only 360 of the extra 723 home runs.

Breaking down the league by age, we see an even more notable spike elsewhere. Older hitters saw a surprising rise in home runs as well. Even those who should be well into a decline in the power department.

The home run totals for players 31 and older have steadily trended downward since the end of the steroid era, considered to be 2006 with the introduction of MLB’s drug testing reforms. In 2015 however, those veterans hit 320 more home runs than 2014 while posting the highest year-over-year increase in home runs per plate appearance for that demographic since the 1993 (plus-24 percent), 1994 (plus-18 percent) and 1996 (plus-22 percent) seasons.

A sudden and rapid appreciation by this group is unlikely, to say the least. The findings of MLB aging curve studies are consistent: players get worse as they get older. In a study by Jeff Zimmerman of FanGraphs, players from 1995 to 2005 showed a steady increase in home run production before tapering off after their 30th birthday. From 2006 to 2013, the curve changed to a consistent decline that began closer to age 25. In other words, in the post-PED era, a player’s power production is immediately on the downswing after he hits his 24th or 25th birthday.

This wasn’t the case last season, when the game’s elder statesmen were turning back the clock. The poster child, or geezer, for this group is the New York Yankees’ Alex Rodriguez, who turned 40 last season but hit 33 home runs (5.3 percent of his plate appearances) after missing the entire 2014 season for violating the league’s performance enhancing drugs policy. In the preceding three seasons, he managed homers in just 3.6 percent of his plate appearances from 2011 to 2013 combined, never hitting more than 18 in a season.

Age 31-plus players Joe Mauer, Ben Zobrist, Robinson Cano, Shin-Soo Choo, Torii Hunter, Ryan Howard and Albert Pujols all saw year-over-year improvement in terms of home runs per plate appearance. Pujols, at 35 years old, hit 40 home runs in 2015, his highest total since 2010. Nelson Cruz, the league’s sole member of the 40-home-run club in 2014, hit 44 home runs in 2015 despite making less contact, pulling the ball less and hitting fewer fly balls. He turned 35 in April.

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Is it the preparation?

Those final notes about Cruz’s season add another league-wide layer to the mystery, regardless of age: While more balls are leaving the yard, players aren’t making more contact, nor did we see an increase in the quality of that contact.

Scott Spratt of Baseball Info Solutions, a firm that provides data to major league baseball teams, has video scouts designate every batted ball as either soft, medium or hard.

In 2013, 30.5 percent of batted balls were classified as “hard,” 29.1 percent in 2014. So, the 29 percent of batted balls classified as “hard” in 2015 is a continuation of a two-year decline. In addition, there were fewer fly balls hit last season (33.8 percent of all batted balls, the lowest on record), yet more of those fly balls cleared the fences for a home run.

Perhaps one explanation is the approach of modern hitters. In part fueled by baseball’s analytic revolution, teams have deemphasized the negative impact of a strikeout, encouraging hitters to swing hard and for power. One MLB executive, who spoke on the condition anonymity, theorized that emphasis has helped breed more power hitters in the game, particularly among young players.

“Organizations are valuing guys with a specific type of swing and approach,” the executive said in a phone interview. “That’s why when solid contact is made, there is higher bat speed and higher exit velocity. That’s why you are seeing a home run-to-fly ball ratio that is a little more significant.”

Combine this approach with better information and hitters are better prepared than ever to smash a home run.

“I’d put it [the home run spike] more on the advancements we are making,” the executive said. “Teams are smarter, more information is available and there are philosophical shifts happening all over baseball. We have the tools to analyze everything now and we are valuing things differently.”

Harder swings combined with better insights regarding pitch placement and type would definitely impact home run rates. As would the trend of hitters willing to take a few strikes in order to key in on a pitch they can crush.

“It’s a mindset,” said one MLB scout. “Guys are taking more pitches, getting themselves into counts where they are trying to drive the ball. There is a concerted effort on the hitter’s part to do that.”

But why is the home run spike only coming now when the advancements in scouting reports and the emphasis on power over contact have been in the game for years?

“Guys that don’t have those specific characteristics are being filtered out of the game,” the executive said. “And the cream is rising to the top.”

Maybe it’s a little bit of everything?

While no single factor provides a clear cut answer for the home run surge, the best explanation may be a perfect storm. Combine the increases due to park effects, warmer temperatures, a generational rookie class, better informed hitters who are swinging harder than ever and the paring down of the game’s elder demographic to its best power hitters, and it’s reasonable to believe that combination could produce the 723-homer spike in 2015.

But perhaps there is one more cause. The biggest culprit might be expectations.

After a nosedive in the home run department in 2014, we would expect some sort of progression back to “normal” levels in 2015. In the nine seasons since the end of the steroid era in 2006 to 2014, major leaguers have averaged 4,801 home runs per season. In other words, if 2014 hadn’t been such an abysmal year for home runs and was instead an average year for long balls, the 2015 uptick would have been just 108, a mere blip we’d hardly notice.

Though it may not satisfy conspiracy theorists, a simple progression back to the post-PED era mean, combined with the variety of other factors above, might just be the best theory to explain the single biggest home run spike since steroids swept through clubhouses in the 1990s.