1993 Buick anti-lock brakes.

My 1993 park avenue has 165,000 highway miles and has always been very dependable. Two months ago, I took it in to the shop to have the rear brakes done. Shoes, drums hardware, etc.. Occasionally when braking, the car surges to a stop and the rear brakes actually drag, or lock up. Afterward, the pedal is super good and firm and everything is ok until the next time it happens. Once it scared me and wouldn't release, so I got the car into a parking lot and jammed as much force as I could assert on the pedal and it released. The mechanic has no clue. I cannot lay up the car for several days for him to troubleshoot this problem.