Why are bus drivers so incredibly soft-hearted?

A few days ago, when I was on a bus and it was just about to depart from a stop, an elderly woman in her 70s did something I've rarely seen—instead of knocking on the front door, she knocked on the back door.
The driver hesitated to depart but ultimately decided to open the back door for her so she could board. Why would he go to such lengths to be considerate?
Even though she knocked loudly on the back door, it was my first time seeing someone being picked up through the rear entrance.
When the doors are closed and the bus is about to depart, it's usually difficult to open them even if someone tries to board through the front door properly...
What is the reason they absolutely insist on opening the back door rather than just leaving like that?