Ok. I've had a theory most of the afternoon. I've held back speculating on it to be respectful to the situation.
Since others feel compelled to try and make an argument at odds with my own, I feel compelled to present my alternative theory.
For my two cents, i say: a raft had just come off the lift and was stopped by the gate control system in the station as the load zones ahead were occupied. The conveyor belt SHOULD have stopped preventing further rafts from entering the first load zone. I'm not sure whether it is computer or manual control. Either way - the conveyor DIDN'T stop. When rafts enter the load zone, they do so on a downhill angle, slowly floating to level as the rest of the raft leaves the conveyor. In this case, (in my opinion) with the first zone occupied, the raft has come down on top of the raft in front, which of course is held in place by the control gates. The conveyor continues to push the second raft forwards, pushing the back end lower and eventually underwater, which in turn raises the front end. Eventually it flips, and there you have it.
Nothing to do with riders not following the rules, which I will admit was my first thought when I heard the news. After reading articles, looking at the pictures on the news, and talking to a few friends with a bit of industry knowledge, that's where i'm at.