i encounter this when having a severe case of blue balls. feels like i need to go to the toilet, but i dont. also cumming doesnt immediatly help in this state, for me it gets a little worse and then better after a while.

but maybe its something totally different

The mens equivalent of the kegel muscles will engage you lower abdominal muscles when you strongly contract them. This usually only happens when the pelvic floor is pretty weak and/or the person isn't very on tune with what their body is doing. It could also have been his sexual form and body movement during the act.

Regarding your last point, exercise in general tends to stimulate the bowels to move.

Never happened to me. I would talk to a doctor.

I've never had that happen. It does happen with blue balls on occasion. But ya know, I have hands and can take care of myself if need be, so don't let guys use that as a pressure tactic.