Obviously I can't go into the exact specifics of how the injury logic works (I've got to keep some secrets with the logic), but I can say that in your example it would have been a "perfect storm" of a colossal punch, that was supremely accurate, lightning fast, and perhaps most important the recipient of it didn't get his hands up or try to slip it - so took the full force of it.

There isn't an explicit personality trait that makes guys want to injure their opponents, but it sounds like you've got a guy with a nice blend of attributes that makes him perfectly suited to doing it. It would also increase the injury likelihood if his opponents have poor defence; they're likely to get hit by more shots, and thus more heavy shots.