When I brought my mom to the doctor because of her persistent back pain, he asked her where it hurt most and then he actually pressed on the area himself, as hard as he could! My mother was almost in tears because it hurt. Why on earth would he do that?
It must have been a surprise to have the pressure put on the painful area - it might have been a good idea for the doctor to warn your mother if he didn't. That being said, what her doctor did isn't unusual.

When a patient comes in with lower back pain that has no obvious or diagnosed reason, it's hard for the doctor to give a good treatment plan. If he or she can get a better idea of the pain, then managing it can become more realistic.

In your mother's case, it could be that the doctor was looking if the pain was local (right at that area) or if she has referred pain, which means that if he presses on one spot, the pain is worse in another.

By knowing this, the doctor has a better idea of what is going on.