My last job was physically strenuous. I took a new (desk) job at the end of June and have since acquired a shiny new belly. I'm not sure where it came from.
Regarding belly gains - my guess: you had a higher basal metabolic rate (BMR) during your last role given it was more physical. You have since reduced physical activity (exercise) and continued eating the same amount (or more). All the while your BMR is decreasing, and so you have gained weight, which for you is stored on your belly.