Looks like the back end got loose, he hit the brakes, which caused even more weight transfer, and the thing did a 180, tossed him off the back and then the bike flipped repeatedly. I couldn't see if he got hit by the bike.
IMO it was f'ing stupid going that fast in that terrain on a loaded, heavy bike. He's damn lucky it wasn't a lot worse.
'12 Suzuki V-Strom DL650
'96 Suzuki DR650
'10 Kawasaki KLX351S #320