Okay, here's some reasons why you may come more than usual.
1. Drinking lots of water about an hour before you do the deed.
2. Holding off (prolonging) orgasm (hence "stop and go")
2. Being extremely aroused (at jackinworld, many guys testify that they came with more force or semen whilst in a sexual situation; that is, they came more if their girlfriend was watching
I hope that answers it. The first two reasons ALWAYS WORK if done properly!