I've battled this in our house every room I remodel. The original oil based primer coat has deteriorated and if you disturb the paint at all, like update molding or strip and repaint baseboard radiators, it chips off all the way to the primer layer. Some rooms have 6-7 layers of paint. My house was built in 56 and I've tested for lead every room in the house, no lead. But the issue makes it easy to scrap off all the paint, sort of. Then I clean, skim coat, sand, clean and apply a new coat of primer before repainting with color. I usually screw down the drywall too and spackle those holes, since I've had a lot of nails pop. It's a huge PITA, but the results are worth it I think. The walls look way better without the orange peel texture of 6 layers of paint and I don't have the chipping and nail popping problem.

TLDR strip walls, fix defects, sand prime paint