I started to binge/purge when I had overeaten, so that would be my number one reason - overeating or eating things that I didn't want to.
Then, because of that, i'd feel fat, sad, angry with myself, stressed, depressed and felt like I was the worst person in the whole world. Those became reasons for me to do that.
But, then again, it just became a parte of my routine, either I felt bored, excited, if I missed the gym, if I could fit in a small size, etc.
It'd would be a way, as some of you referred, of self-punishment.