The why is simple.
Because he can. This sounds like someone who is quick on the read.
He assessed you as a victim typology very quickly and continued with a behavioural pattern he knows /has grown up with.
Most likely aggravated by an alcohol problem.
And he will do it again.
I am curious about couple of things.
What stopped you from calling the police.(this isn't a criticism, any of us who have been in an abusive relationship have done it, I am wondering what your reason was.)
Can you remember what set him off?

Please be careful and take care.