This is what happened to me. The rear brake pads are shite. They will flash rust (just like the rotors) and get stuck to the rotor. Remember the parking brake does not use a separate brake pad like most do and when moisture gets in there the brake pads get rusted together with the rotor when the parking brake is appied. I only drive my car on the weekends with a wash at the end of the weekend. The car was not driven for 2 weeks prior to my brake pads rusting to the rotors on both sides.
I replaced the pads with Hawk ceramics and had to remove the old brake pad material that was rusted onto the rotor and all is good now. The brake caliper was not seized, but rather the pad was rusted to the rotor. If the OP's problem is the same as mine, more than likely the problem will happen again since Ford will use the same shite brake pads. Interesting though, the fronts have been problem free (so far). Also, in 10,000 miles this has been my only problem with the car.
2011 GT Coupe Premium, Black exterior with Charcoal black turned leather interior. Rapid spec 400A with 6-speed MT, 3.73 limited slip axle, 19" painted aluminum wheels, HID headlamps, comfort package. FRPP tune with K&N air filter. FRPP Touring axle back exhaust with Borla 2 3/4" over axle pipes. ST Suspensions coilovers w/GT500 mounts. Spohn poly/del sphere adjustable lower control arms and Spohn adjustable panhard bar with Steeda panhard bar brace. Hotchkis Sport sway bars.