Looks like from the sound of it, some people tried to lay back and jump the restart to get an advantage, but the lead guys hadn't taken the green, so they were still doing pace speed... When the jumpers realized what happened, they tried to slow up, and got plowed into, setting up a nasty little chain reaction.
It might have been the other way around though... Leads tried to jump the green and then checked up when they didn't get the flag, and it was a chain reaction behind them. Not too sure, but those seem to be the two versions I got out of reading.
What a bunch of idiots. It seems like they don't have very strict rules on this kind of thing from what most people are saying, and that's a shame because it caused a heck of a disaster.