OP: is your bike in a workstand? If so, follow the rear brake cable routing to make sure the cable isn't being clamped against the frame where the bike is clamped into the workstand. This sometimes happens and the cause isn't immediately apparent.
I can't think of anything beyond what's now been suggested that would cause them to lock up like that. For experimentation purposes, you could release the cam lever on the brake caliper; that would loosen the brake somewhat. You might then be able to get the wheel out so you can have easier access to the brake caliper.
Good luck!