I've seen something like this but not to this degree, dude was severely dehydrated but he at least made it out of the round before he passed out. That's what this looks like to me. Seizure due to severe dehydration maybe trying to make weight and didn't properly rehydrate. Dude was probably seeing stars and everything.
Dehydration. I think that's a cause for most deaths in boxing. No fluid in your head so your brain is just bouncing off your skull.
It's why I'm opting for Muay Thai. Still dangerous, but you mostly take leg kicks.