It's called Experience!
Many Passengers, enter the bus and try and stand in the doorway so as to be able to be first off the bus when it arrives at the terminal. Thus obstructing others trying to get on.
First bus then pulls away not quite full and 2nd bus gets filled to capacity with still a few stragglers unable to fit on the 2nd bus. Therefore a 3rd bus must be called with crew and staff tied up waiting for this bus with stragglers causing a longer delay.
By waiting until both buses have the complete passenger load on board a 3rd bus is not required.