Of course it's unbelievable, and it hurts like hell. He is
pissed, and you are going to be the whipping post, he is giving up his best friend, he is a mess, detox is brutal, he doesn't know what he feels, the only thing he knows is he wants to bug out, he wants every one to hurt like he does.
I hate to say it, I mean really, why would anyone who is going through what he is going through right now even logically think that a life changing decision like divorce can be made. He is probably trying to figure his way into his next drink.
You need to take care of yourself. Alcoholics always throw the unbelievable at us. My axbf was railing at me one night, we were sitting outside and he was pointing his finger at me, telling me I was stupid, evil, everything was my fault, on and on ad on and the next day, told me I started it, huh??? I said not one word, not one. About an hour later I was the best thing since sliced cheese. It's madness, all of it, It will never make sense and it will change, right now you are the object of his anger, you can sit there an take it like I did, or you can tell him to keep it to himself, of course he isn't explaining anything, there is nothing to explain , except that he is using you as a whipping post.
I'm so sorry you are going through this, you did nothing to cause it.