Who cares why he does it. The point is he does it. All that should matter to you is 1) does it bother you and 2) Is he ever going to stop?
There could be no logical reason at all. A shrink could sit down with him for months and unravel an elaborate reason or trauma from his childhood where this is all rooted. But frankly who cares if all he does is nods at his shrink, drive home, drink, and acts bilegerent towards you. Understanding the problem isn't necessarily going to solve it or make you feel any better.
Dump him. Or when he does it just walk out the door and go have fun until he's sober.
Deep thinking is overrated. The simple approach is often best.
you have to bear in mind that when someone drinks they lose their abilty to kinda "filter" out what they're thinking,and what they're saying,so as awful as it is,he might actually be thinking the drunken slurs he says to you all the time,but just doesn't say it when he's sober.