I would say lack of maturity and selfishness. My ABF used to do this initially, then a year or so later it escalated to him going on benders alone when he became unemployed. Then more recently when he quit the benders when i'd told him they were unacceptable, he started drinking vodka during the day. He is now 37 and I too felt he should have grown up. I am 39 and had grown out of drinking excessively so couldn't understand it. He also appeared to be able to drink normally on some holidays or if we went to dinner so I didn't see it for a long while.
Now, I'm not saying your boyfriend is an alcoholic or will do the things my ABF did but I would watch other behaviours as a clue - my BF has other addictive tendencies (used to smoke 50 to 60 cigarettes a day before he gave up, can't stop eating certain foods he likes, drinks water like a fish, works all hours) which I humoured until it clicked that he is an all or nothing kind of person and where alcohol is concerned, I think it needs to be nothing. Sadly, I'm not sure he agrees so he is now moving out.
Have you explained to him in the morning that this behaviour is not acceptable? What does he say?