That’s known in the trade as a tank-slapper. That particular one was caused by a combination of the front wheel landing crooked and the rebound of the suspension. It would probably have damped itself down even if the pilot had managed to hold on, but the violence of the first couple of oscillations was too much for him and the rest of him wasn’t well attached to the bike anyway.
The physics of tank-slappers is pretty complex. Variables include tyre pressure and condition, front suspension, rear suspension, weight distribution, road condition and usually, acceleration and probably a bunch of other stuff.
They’re thankfully pretty rare because most motorcycles aren’t ridden hard enough to provoke them.
Here’s a brief introduction to the concept by Fortnine…
