To the OP, please post some good pictures of which brakes you have to stop all of the quibbling. This will help us help you. I would look for things described above in addition to a possible leaking caliper or hose getting fluid on the rotor, a possible leaking grease seal getting grease on the rotor. Grease or fluid will make brakes grab and lock up. But again, pictures would help the situation. You're obviously going to have to remove the wheels to inspect and fix the problem anyway, so get us some pictures up. Thanks.