The decision to open a flight (not the door, but the flight) is made by Ops, not by the GA. It isn't that common, but it is far from rare and it's based on lots of factors, not the least of which is whether the passenger can be added without delaying the ultimate departure of the flight. If there are other ground delays, no reason not to take on the extra passenger as it won't change the arrival time.

Conversely, the seat may have been given away and it's tough luck. Returns to the gate are less frequent, but they happen. Not likely on a flight such as this, but last flight connecting to a TPAC for an EXP in paid F and it's more likely. Same thing for crew as "must fly".

Adding the passenger means that weight & balance needs to be recalculated and transchecked by dispatch and the crew. That can take a few minutes, so it isn't just a question of the 15 seconds it takes to pop the door and close it again.

Needless to say, it's a sore point because on your next flight, you run for your connection, you arrive at the gate at T-14 and it's too late and too bad.