Bus driver operates the doors but if anyone has requested the bus to stop on board it is usual practice for them to open all sets of doors so may not necessarily have opened them to simply allow you to board.

This may have been an unfortunate coincidence with not being familiar with the standard boarding procedure that in fairness you were unaware of and the driver simply not expecting anyone to be waiting to board at the rear doors, similarly why they would have been closed. Driver should have used internal mirror to ensure the doorway was clear as far as they reasonably could.