Nobody actually knows. He had that only on one side of his face, the other side looked normal.

Some of the theories is that it's congenital (basically he was born like that) or that he had so sort of procedure that led to that (botched plastic surgery, removal of salivary gland, some sort of maxillary surgery etc).

The funniest theory I've heard is that he was actually Hitler and that's the part he left in Berlin when he tried killing himself.

He has RoboCop unmasked vibes. Instead of a human face on a robot body it's like a human-ish face bolted onto a shit factory.

At least we can all discount it being caused by combat.

Not sure if it is the case here as another comment said it could be congenital, but multiple facelifts also cause this. You can see it in some older actors/famous people, although recently (last decade or so) it's been better 'hidden' with the use of fillers.