Looks like Graham hit a drainage ditch just before the railing, that appears to be what launched him over the rail.
Nothing like a little wet grass to get you right out of control, I've hit it a couple of times myself during my years as a speedway racer.
I did have one rather convenient crash years ago though, lost it on turn 4 entering the main straight, hit the grasss, then stopped against the side of one of the track ambulances.
IF you gotta crash, try and do it conveniently.