Cause of paint chipping?

My mechanic accidentally chipped this section of paint near the headlight. When I pointed it out to him, he said it was due to the bumper not being primed properly; I had the bumper replaced by a body shop in the past. He said that he’d get an estimate from his auto body guy and would like to split the cost of repair with me. He stated his intention was to repaint the entire bumper. What would be the cause for the fractures at the bottom? Should my body shop have primed the bumper like he said?