ppl are saying either
1) because he was consciously planning to do it to report him for abuse.
or
2) he had a seizure or breakdown of some sort.
I've been this dude. It's kind neither. If it's either it's a bit of 2) but it's more of a boiling point of all the frustration and aggression that you carry with you 24/7. Moment to moment you don't consciously register that those feelings are there, but they are. And then you're physically held down and you're pushed to your limits and you crack...all that pain and rage from all your anxieties come exploding out of you and you just wanna take it out on this person who's forcing you through it. But you know that you can't, and so you have to get it out on the nearest motherfucker you can so you can beat that stupid motherfucker's face, for just coming near you while you're at this point. How dare this motherfucker come across your wrath. And that motherfucker is you.
So you spend the next however long it takes to thrash your stupid fucking skull against the wall until your tired aching body stops.