Rear drum brakes can lock for several reasons. One reason could be a defective wheel cylinder, which is part of the brake system. It is found at the top of the shoes, and its function is to place force on the shoes so they comes into contact with the drum. A defective cylinder can cause brake lock up. Your parking brake cable could also be adjusted too tight. The slightest pressure on the brake pedal, will then cause the brakes to work at full force, causing the lock up. Loosen the cable a bit, in order to relieve some of the tension on the brakes, and avoid them locking up.