From personal training and counseling many clients a lot of it is depression or traumatic event combined with negative coping habits and routine or just poor routine.
For those who didn't go through having a baby or experiencing a death or grief, car accident, they just had a poor routine of going to bed, waking up, going to fast food before or during work and before they know it, they are large.
Drinking lots of alcohol and eating fast food will blow your ass up. A lot of the bigger people I know hunt, jet ski, and fish like every other day but are still big because they pound beers all damn day on their weekends.