I completed A Night To Remember on my character a very long time ago. Over 300 in-game days have passed easily. I went back into Morvunskar just now and after entering I hear a lot of Fireballs being thrown around. I rush through to the end and see Sanguine battling three enemy mages.

Is this normal?

Try talking to him and see if he's drunk.
Wait... Sanguine is always drunk.

Well, depending on what you mean by normal: no, Sanguine shouldn't be there, but yes, many seem to have experienced this. I've never checked Morvunskar after the quest, I might do so and see if he's there.

I've never gone back there but other players have said that when they did, they encountered the same thing as you. Apparently he does hang out here and I guess maybe the enemy mages respawn after a while. UESP says he can sometimes be found there fighting them and this is possible any time after A Night to Remember has started.

Get behind him and go into sneak mode so you can check out what's in his inventory.

After one of the patches he's often there. Shortly after this started happening people (and I thought Wikia and UESP) were saying that he was there so you could exit Witchmist Grove if you went back in after the quest had been completed, but now Wikia is saying you can still get stuck there. :S