Late Tuesday in the US, the bottom fell out of US stock market futures contracts.

S&P 500 futures were down by as much as 1.3% at about 9 p.m. ET. These particular futures contracts are among the most widely traded securities in the world.

It's also worth noting that this isn't like Twitter or some other single stock that might see big intra-day swings. S&P 500 futures contracts are tied to the S&P 500, and index of 500 companies. So a 1% move in a matter of seconds is a big deal.

"Selling increased after the contract slid below its average price over the past 100 days, with volume more than twice the five-day average for this time of the day," Bloomberg reported.

Analysts were left scratching their heads.

The move happened as China's official manufacturing PMI report was released. The headline index unexpectedly climbed to 50.1 in March from 49.9 in February. Anything above 50 signals expansion, so this wasn't terrible news.

"I can't point my fingers to anything that would have triggered this," AMP Capital Investors' Nader Naeimi told Bloomberg. "I guess that's how big corrections often start."

That latter part is scary. And it isn't just fear-mongering.

Check out the chart to the right from UBS' Julian Emanuel. As you can see, big crashes don't just happen. They're wild processes with big swings.

"The manic market movement — sharp plunges and even sharper surges — is not unlike the movement seen prior to major market tops in 2007 and 2000," Emanuel wrote in January.

Futures have recovered most of those earlier losses, so it doesn't look as if Wednesday's regular trading session will start too deep in the red.

While not everyone is predicting a major downturn, most stock market gurus warn that more volatility is coming.