It's called CTE.
Chins (or more correctly the ability to recover from a concussion) crack at different intervals for different people. One fighter might get blasted 99 times in his career, and then on the 100th his ability to recover from concussions is simply gone. Another fighter might get blasted 9 times and on the 10th his ability to recover from concussions is simply gone. That is how CTE works. There is legit science to back it up.
Luke Kuechly, NFL linebacker, recently retired because he is no longer able to sustain head trauma at the age of 28. His brain is too scrambled from all the hits. Almost every time he tackles a guy now he goes unconscious.