From what I can gather, when you've been fat in the past, the little fat pockets/cells never go away, even when you've lost weight. So while you can lose fat and deflate them, they're still there waiting to inflate your arse again. So when you have a few weeks "off the wagon" so to speak, you'll find your weight can spike massively with water etc. Resulting in a big gain. That's why it's highly unlikely to be all fat gain, and normally you can lose it again quite quickly.
That's why skinny people don't seem to have an issue, and us fatties always seem to look at a chip and gain 20 stone.
Sorry if I haven't explained this very well but you get the gist hopefully