If your parking and leaving the E-brake on then i'd say thats the trouble. Through an extensive period of time the car sets, the Brake will try to fuse to the drum thus making it lock. I use to do that very thing parking using the E-brake but dad always told me dont do it becouse of that, so i started parking and leaving the car in reverse gear. If your on a slight incline, use the reverse gear and block the wheels. If your on a steep hill, only then would i use the E-brake and THEN leave the car in reverse and also turn the tires toward the curb. Also blocking the rear wheels.