I guess it can't be too much food and not enough exercise. :) I worked on a freight crew at a Home Depot after getting laid off of a programming job. I was in great shape, but still slightly overweight. After finally getting back into my old profession, my weight went back up with my salary. :( So now I spend my lunch breaks on a treadmill.
That's my story too.
I took a low-wage retail job at a big box precisely because I needed to get into shape. Nothing wears you down physically like retail, (well, maybe construction, I don't see many fat construction workers) so it's inevitable that you get into shape. Being in shape made me more attractive at job interviews so I eventually got a real job. But after that, back at the computer, it was the battle of the bulge all over again!