Abusers seem to abuse because it gives them a sense of power. They tend to be insecure, in general, and they use any perceived infraction or sleight by their wives and girlfriends as an excuse to abuse. They are proud of abusing, believing that somehow this makes them more of a man. They will blame it on their women, on nagging, or anything else.
So I think it comes out of insecurity, and the ability to physically control the woman, which makes them feel better about themselves. These guys have shorter fuses. They are disagreeable and unforgiving, and will pop off about anything. These men need power and control, and will use women to get those things.
This article describes a study designed to answer this question. Here is an interesting quote from the article:
In fact, Ou found that unquestioned obedience to authority defined abusers more than their attitudes toward women or wife beating.
“Abusers are less forgiving of those who violate law and order and more punitive to those they consider weak,” she says. “They favor harsher punishment for lawbreakers. They feel little compassion for those they regard as inferior. When they treat women as inferior, sexism becomes a subpart of their authoritarianism.”
It seems that marriage makes the severity of the abuse worse, perhaps because it’s harder to get out of a marriage than out of other kinds of relationships.