That because the alcohol affects reasoning and judgment as it saturates cells of the brain and causes vital neurotransmitters and proteins to not function as well as they normally would. It causes a loss in inhibition which contributes largely to their stupid behavior, and they stumble because the cerebellum, which controls balance, is affected by the alcohol it is bathed in.
All their behaviors are caused by two things, A) because their neurons are now bathed in alcohol and don't function as well, and B) people are actually inclined to act stupider in social situations like that as it's a learned behavior. They feel like because they've gotten drunk and acted stupid before, their brain thinks that's the behavior that goes with all situations that are of that sort, so even if they're not that drunk they can still act stupidly because of that subconscious learned behavior complex.
I read some where that Psychologists have done experiments where they put a group of people in a social situation where alcohol would be consumed (such as a bar) and then give them nothing but alcohol-free drinks and they STILL act like they're getting drunk (ie talking louder, slurred speech, freedom of inhibitions, moving closer and becoming more interested in the opposite sex etc).
This simply happens because their brain thinks that's how they're supposed to act in that situation, regardless of whether or nor alcohol has been consumed. It's actually quite an interesting area of psychology.
EDIT: I forgot earlier, they can also wake up with headaches as if they're hungover from such an experiment. The subconscious mind really plays quite a large part in determining behavior and how the body responds to certain situations.
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