Here are my 2 cents: He was never into resistance training, he has stated that he doesn't like doing deadlifts e.t.c. thus his ligaments, bones and tendons were never really used into withstanding big loads. Muscles are easy to train and adapt faster but ligaments, tendons and bones require longer periods to adapt.
Punk aged and came back, he is no longer 35 years old and the rust is indeed there, his matches in AEW weren't bad but you could see that he was taking is slow and easy in most spots.
From what I've observed during the later days of his AEW run he probably used some kind of substance which increased his muscle mass, I think he used more of it and he was even buffer in his Royal Rumble WWE debut, his most likely is the reason he got injured: His muscle mass increased more but his ligaments/tendons couldn't take the impacts and thus he was injured.
