For me the culprit in sudden weight gain is too much protein. I can lose on 60g/day, maintain on 70g, but more than that I gain.

An example: The first week of December I spent 4 days out of town. In my efforts to eat cheap (not at restaurants), I ate more protein than usual. When I got home I found I'd gained 4 lbs. I got back on plan & by the end of the next week I'd lost all 4 lbs. Water weight? Maybe - I don't know. But it was a relief to see it go!