rear brake eating up rotor

[p00kienrayray]

I checked my rear brake rotor today and I found grooves on the outside, while the inside is fine. I look at the pads, there's a good amount and they're even on both sides. I haven't had time to take the calipers off to look at it, but any ideas of cause before I start on anything?
btw the front rotors are fine and I use the fronts more than the rear anyways.