Wonder what really causes this?
In drew dixon's case, he's just a drinking before filming and literally pissing himself while getting fucked. It's so clear it's planned.
When it happens "naturally" is due to your bladder or urethra literally getting the pee squeezed out while fucked.
You know how you can control your pee stream? Well that's an actual muscle you contract or let go. It can be "reached" anally and also mentally one can involuntarily lose control of it due to stimulus such as fear or arousal.