I got stuck on the Gemini lift once and they sent up two different people just to talk to everyone to keep them calm. I believe at one point they did say that if we were to be evac'ed it would be front first. Of course, it was actually just a false alarm and the ride was fine and it ended up we had sat there for two hours for nothing. On the bright side, we won the race... Not like that really matters. I'm not even sure Gemini is equally matched.
According to the incident report, the train started to move due to the weight shift as Hix said, but what the video doesn't show clearly is one of the staff members wearing a harness secured himself to the train.... he jumped into one of the empty seats as it started to move.