I see fairly clear, flat ground. I see the brake light come on, 3 seconds later the bike starts to fishtail. There is a tank slapper and the bike tumbles for about 3 seconds. This doesn't look like a terribly fast crash, perhaps less than 50 mph. I'm guessing by how long the bike tumbles.
Why did he hit the brake? Why did the brake light stay on even after it looks like the rear tire is locked up and sliding? Who knows? But to call the rider stupid is absurd. Depending on the rider's experience all sorts of reflexes can happen; some good, some bad.
You know what stupid is? It's swinging your leg over a motorcycle and riding away. Don't you know it's a lot safer to travel in a cage? Ask your mother, she'll tell ya.
If that was me in the video, and I found that the little rocks didn't deflect the tires too much, I'd be trying for a land speed record.
I'm very happy the rider seem in good shape and spirits.