Can you push the pistons back easily with a screwdriver? If so they are not seized. The pistons in the rear caliper only have to move back a few thousands of an inch otherwise you would have too much travel on the rear brake pedal when trying to engage the brake (since the pistons would have to travel the extra distance to cause the pads to grab the rotor).
Some reasons for dragging brakes:
- as Canberra stated.... pads too thick (were they just changed?)
- warped rotor
- bad wheel bearing
- loose axle nut resulting in a loose/floating wheel
- cracked bearing bore in rear wheel
- seized caliper on pins, or seized pistons
- sticking master cylinder
- dragging foot on brake pedal
- rotor too thick (ie non oem)