That's a short clip of a longer video...the longer version was clearly fake but I can't remember exactly what gave it away. I thought it was posted here but maybe not.
I've seen a similar save in a crash montage posted here a few months ago.
A guy fell off the back (trying to stand on it while riding) and the bike stayed up and kept going and one of the onlookers yells 'catch it catch it!' and someone managed to jump on before it fell over.
That was a straight line at pretty slow speed. The OP's vid has the guy falling at the end of a turn which has me skeptical but I'm no expert...
Something is funny about the way he gets up and the other bikes coming at him. Its like he jumps from one side of the track to the other as he is running towards the bike.
I say it's fake. He would have rolled like a stone if he fell off at those speeds, and I doubt he would have been running any time soon. His dismount looked contrived as well, but the bike could well have kept going by itself for a bit.