What made the bike fall??
Flat tire??
He was upright when..., boom, he just goes down.
Anyone who rides a motorcycle will tell ya, it just doesn't happen like this. You usually go down in a slide or speed wobble out of control.
He just drops while upright. Hold a bicycle tire by the axle, spin it real fast, then try to turn it on its side..., You can't.

Suggestions as to what caused this...?

Watched it 50 more times (the Crash part)
There is some smoke just before he goes down but still no indication as to the cause.
Flat tire, Engine seized up ??

He braked coming behind the other guy.....lost traction and his front tire slide right. Never brake while turned...especially that fast on a bike.

His talent ran out before his ambition did.