Cain has had 4-5 back and knee surgeries, 36 y/o and very long inactivity. He very well could be completely shot.
Wasn't a punch it seems, but just his body that gave up. He should just call it a career. More than 2½ years out with injuries, comes back, and his knee gives out after half a minute. Gotta feel bad for Cain, injuries ruined an potential all time top 3/5 legacy.
And in the last replay it did look like Ngannou landed a clean uppercut that buckled Cain before his knee gave out. I think it was a combination of that.
Cain's knee gave out, but these ref's have to stop ****ing fights up. It's a fight. Let a fighter get clobbered a few times before stopping it. That is the second Ngannou fight in a row where the ref stopped it too soon.