Ford Explorer brake oddity

I'm doing a brake job on my neighbor's '98 Ford Explorer. It's his wife's, she heard a noise that she thought was the squealer on the front disc brakes. We pulled it apart and the pads were just about half warn, not nearly bad enough to need changing, but we changed the pads as long as we had it apart. He drove it down the street for the road test, same noise. I was standing on the right side of the street and didn't hear it. I moved to the left side of the street and heard what sounded like the normal gravel dust on the pad sound. I pulled it apart and found that the pad was warn into the metal and had gouged the rotor pretty bad. I don't even know if there is enough left to turn, I doubt it. I pulled the other side and it was good, hardly warn at all.

So what would cause the left rear to wear so bad? The parking brake is drum type built into the center of the rotor, so it's not a frozen e-brake cable.

BTW - these are some of the easiest brakes I've ever worked on. The only thing I remember easier was a Triumph TR4. I want a set on my Jeep.