I'd say its a mix of everything one's sad above: slows the metabolism, causes drunk munchies, high in carbs and sugar..... but the one that really gets me is the water retention.
Now that I've been on track for the past 4 or 5 months, I've drank twice I think and each time I put on about a pound or two in water weight that came off within two days as long as I went right back on my plan (and even after I allowed our late night traditional Whitecastle run, surprisingly).
When it all comes down to it, I think it's okay to have that fun night every now and then.... the little bit of weight and/or water weight you put on from that one night will come right back off if you stick to your other plan.
Does for me anyways.