Getting slapped in the face with a penis

So this is my first ever “post” or whatever so bear with me... I really would like to know why when you are giving a blowjob, as one does, to a guy he feels the need to slap you with his peter. I was talking to my mom and sister about it last night (we are very very close) and they said that almost every sexual partner they have been with has slapped them with it. Does anyone know why that is?