Sooner or later it had to be "squeezed" in. Due to the overcrowding on buses, it's inevitable that this will put a hard strain on the bus driver. Without excusing some's erratic behavior, they probably do feel much stress. Sometimes, not to say very frequently, they must deal with MTA's mismanagement's consequences. They must manage excessive overcrowding, and those who are polite and don't wish to offend customers, probably find it difficult to balance the need to please the customers with drawing a line somewhere.
Sometimes the driver is already operating a packed bus, but still tries to let more people in as if trying to assist as many people as possible. That causes a tremendous stress. The bus is frequently so full that it's a test for the driver's nerves to deal with it, and some drivers do it very well.
More on this later, as there are some good examples to draw from actual scenarios.