For some, it's from a bad diet from childhood that carries on into adulthood. Or you slowly eat more and more, until you get to the point of eating entire boxes/bags of food. You're so used to it you don't even notice how much you've eaten. For those who deal with it, it's terrifying when you finally notice.
Not firsthand, but my mother. My brother too is starting to gain weight. Being the smaller one in the family, I'm scared.