Or the caliper slider bolts being sticky, and not allowing the caliper to self-centre and release the pad from the brake disc.
One thing you can check:
*with the car on a flat surface and ingear, engine off (ie: no chance of car rolling away)
*jack one side at rear/axle stand: only need to do this if you cannot easily grasp the rear caliper through the wheel spokes
*release handbrake (hence, ensure car is secure/cannot roll/move!)
*hold the caliper and NORMALLY you can move it very slightly (millimetres) side to side (left/right); this occurs becaue the pads are released away from the disc (leaving a small gap), and the caliper can slide sideways on the slider bolts
If the caliper cannot move, then it is seized in position the reasons discussed.
(Ensuring that corner of the car is on an axle stand) you can also put your head around the back of the disc and check its surface. It may show excessive wear if the inside pad is continuously in contact; or this may occur on the outboard surface. The opposite side may not have any wear and be dirty/rusty, if that pad cannot reach and make contact. These are other sure signs of a caliper that is seized in position.
Take it to a mechanic to fix if this has happened. The slider/caliper bots need to be remove and greased or replaced. A stuck piston (or sticky internal handbrake mechanism) will also need fixing obviously.