One of the most puzzling aspects of the 2014-15 Bulls was that the team went from an elite defense to a mediocre one. Ex-Coach Tom Thibodeau always won games via a stout defense - his Bulls teams ranked 1st, 2nd, 6th, and 2nd in defensive rating before plummeting to 11th last year. So what happened?

The Good

Important parts of the Bulls defense were still great last year. Looking at Thibodeau's time creating defenses as an assistant or head coach, there were always certain bedrock principles that were ingrained in his teams:

1. Do not let teams shoot 3's at any cost

2. Don't foul

3. Secure rebounds to finish possessions

Although last year's Bulls struggled in defensive efficiency, they were still fantastic at defending against shots:

stats courtesy of Basketball-Reference

The Bulls remained a top 5 team in taking away 3's, making sure their opponents shot a low percentage on the 3's they did take, forcing opponents to shoot a low percentage on 2 pointers, and not fouling.

As in years past, teams did not take good shots against the Bulls and they also shot a very low percentage on the shots they did take. So why did the defense slip then?

The Bad

Although the Bulls were elite as usual at holding their opponents to low shooting percentages, they let their opponents have way too many second, third, and fourth chances in a given possession.

Bulls opponents saw a 6.7% increase in their field goal attempts per game. The reason for this is threefold:

1. The Bulls' defensive rebounding suffered

For whatever reason, the Bulls really struggled to box out as a team last year. Teams saw almost a 10% increase in the amount of offensive rebounds they were able to grab from the previous season. This must have driven Thibodeau crazy. He was fond of telling the media that rebounding was a key part of defense and that a defensive possession didn't end until the rebound was secured.

2. The Bulls could not force turnovers

Thibodeau defenses have never focused on forcing a lot of turnovers, but good god were they bad last year. The Bulls were 29th out of 30 teams at forcing turnovers and had a 15% dropoff from the previous season in which they were also bad.

The Bulls rarely ended defensive possessions early with a steal. They rarely ended possessions early in the clock. Over a quarter of the time, they also didn't end possessions after the first shot attempt. Add in all of these factors, and the defense had to stay on that end for an awfully long time each and every game.

3. The Bulls played at a slightly faster pace.

The Bulls have always been a slow team, but they increased their pace by 2.6 possessions/gm last year. With extra possessions come extra shot attempts from opposing teams.

The Ugly:

Another area that the Bulls really struggled with last year was pick and roll defense. Thibodeau's system of icing pick and rolls was one of the team's strengths throughout his tenure, but last year was a different story.

(side note: to understand Thibs' defense's principles, Zach Lowe wrote up an excellent explanation for Grantland a few years ago)

The Bulls simply could not stop ballhandlers from penetrating and scoring. The team was dead last in defending the ballhandler in pick and roll situations. Opposing ballhandlers recognized this weakness and went at the Bulls the 4th most frequently out of all NBA teams. This combination of high efficiency and high volume from pick and roll ballhandlers was a problem that the Bulls simply could not fix all year.

One likely culprit is the heavy minutes played by Pau Gasol, star of this and many similar highlights. But an inexperienced Mirotic and injured Noah+Gibson meant, even after jettisoning Carlos Boozer, there was roster-wide issues in frontcourt secondary defense.

Looking at Bulls Defenders' Individual Stats:

For the first time last season, NBA.com's Stats page introduced information that tracked how well players defended shots. They also showed the shooting percentage a defensive player held an offensive player to, and what the difference was in percentage over that offensive player's average shooting percentage.

Here are the Bulls' tracked defensive numbers:

(Note: a negative number = good defender. Positive number = bad defender)

There are of course many flaws with NBA.com's tracking system, but it does give a baseline of how players are doing. The most shocking aspect of the Bulls' tracked defense is that Jimmy Butler nears the bottom of the list as one of the team's worst defenders last year.

Ironically, Butler was named to the All-Defensive Second Team last season despite being subpar on that end by his own admission. He told the media in a February press conference last year:

"I think [the defense] starts with me, to tell you the truth. I'm supposed to be this prime-time defender and I don't think I've been holding up my end of the bargain lately. So I think whenever I start kicking it up three, four notches on defense and not worry about offense as much, I think it'll all turn around.'' "You have to pick and choose your battles and save your energy for both ends of the floor now. I'm not going to lie, I thought it was going to be easier than it is. But to go on one end and produce and then go on the other end and have to stop the best player on the opposing team is not always an easy task."

He reiterated these same comments in April to ESPN's Nick Friedell:

"I'm supposed to be the prime-time defensive guy, and I haven't been guarding a soul. I've been worried about offense too much, and I need to change that quickly or it's going to be my fault."

There is reason to be optimistic about Butler's defense though. He returned to defensive greatness in the playoffs when the games really mattered, holding opponents to 10.4% below their average shooting percentages. Of rotation players, only Taj Gibson was better.

Joakim Noah also faced heavy criticism for his defensive efforts this year. The reigning Defensive Player of the Year looked like a shell of himself at times on both ends of the court. He came off of a summer knee surgery that required him to sit out parts of the season and was forced to play out of position as a PF when paired with Pau Gasol.

The individual decline of defensive anchors like Jimmy Butler and Joakim Noah certainly played a role in the Bulls' defensive struggles as well, along with losing long-time defensive ace Luol Deng in a salary-cutting trade the previous season. Add in the team's collective struggles with rebounding and forcing turnovers, and you find a team that slipped from its perennial spot at top of the rankings into mediocrity.