The door is, after all, only there to keep the draught off - it's the seat belt that stops you falling out.
Gravity plays a role too. Though it's probably best to avoid RH knife edges even with a seat belt.
I was always under the impression that the airflow in flight tended to keep the doors shut rather than open - though I've never tried to open them.
I rather like the idea of bad air, just hanging around on the airborne equivalent of street corners waiting for some unsuspecting pilot to come along so it can trip it up while whistling an innocent tune.