The shape of the takeoff ramp.
it kicked the rear wheel when the front was already in the air.
Sitting down cause the suspension to compress and then rebound, kicking the rear high
the rider should have been standing up to deal with the suspension compression
He also chopped the throttle, if he twisted it wide open, the momentum of the rear wheel would cause the rear of the bike to drop
the dude didn't have a clue, and got bit.