It could also be a heavily warped rotor. You generally don't change pads unless you at least machine the rotors as well, or get new ones.

What could be happening is the rear rotor could be heavily warped, so the pad is only wearing on that "warped" part of the rotor, causing the pad to wear abnormally.

It wouldn't hurt to bleed your brakes and put new fluid in the system, but it doesn't sound like a stuck caliper. If you had a stuck caliper you'd be smelling brakes and destroying pads much quicker than within a year or so.

I once put all new pads and rotors on, and one of the calipers was fouled (and stuck) and it annihilated both the pads and the rotor on that side within about 10 miles of driving, smelt horrible and it was obvious that there was a constant-braking problem. You don't seem to have these symptoms.

So check and see if your rotors are warped, perhaps machine them (should be like $20-$25 a rotor to machine) and replace the pads one more time (you don't want to put old worn pads on a newly machined rotor, or visa versa) and see if that helps the problem.