The rear disc brakes are worn out at 51000 miles, right down to the rotors. The fronts,however, are fine. It seems to me that this situation is backwards. Shouldn’t the fronts wear out first.
Not always the case. Also if you left the parking brake up or its sticking this can be the case.
While the front brakes do most of the work, it is possible for the rear brakes to wear out first. The rear pads are often thinner than the fronts, for one thing. I’ve had two cars that needed rear pads before the front ones wore out.
Didn’t you hear the wear indicators?
Not unusual, same thing happened to my car. Depends on the amount of material available, front/rear balance, etc.