ALOT of factors caused this he was massive for yrs bodyweight,muscle that weighs on your body,the heavy weights of course non stop for decades,then surgeries and continuing to train like the 'king'.
He seems to remember when it happened, I guess, 800 lb squat. He felt it... He then continued to Squat after it happened in that same session.
Which some might think is insane, but I can attest, when you hurt it sometimes, it feels like a twinge at the time and not too serious.
Then you wake up the next day and say "oh fuck, what did I do"?!?