One theory I had was that people can somehow know in advance that, for example, the health care sector would do very well for the day, then they get rid of what they are holding to invest in healthcare, causing most other areas to crash out.
My other idea that I really didn't want to be true was that there was a way for people to manipulate things to go down to hit stop orders, then sell once it goes back to where it was.
If it's the first one, there should be a way to predict it. If it's the 2nd one, well that's just a shame.