What could cause outer brake pad to wear quicker than inner pad?

On my 96 Passat, I started noticing some groaning noises coming when I would brake. Unwisely, I didn't get around to addressing it in a timely manner and in the last couple of days, it became a rumbling and then an awful sound when I would brake and stop.

Yesterday, I dedicated some time to investigate it and noticed that on my rear driver side, the brake rotor (side facing the wheel) was scored (not sure this is the right term, but the rotor was gouged) on the outer diameter 1.5" while the inner diameter and the side of the rotor facing inboard was smooth. I checked the brake pads and the outer pad was completely down to the metal while the inner pad had significantly more pad remaining.

I checked the guide bolts and they went in and out smoothly.

What do you think would have cause this? I am planning on getting a rear brake set (new rotors and pads) and need to see if there is anything else I need to get to avoid this from happening again.

Thanks.

you say the greased rods the carrier moves on are ok, not dry of grease and sticky. if you didn't get enough miles on the pads then the puck being sticky and not retracting when you release the brake pedal. when I say sticky it could mean dried or burned oil or grease coked up hard, or corrosion/pitting. when was the last time the brakes were bled to get old oil out and fresh oil in the brake calipers?

Pads sticking in the carrier. You need to remove the carrier from the car, the small stainless anti-rattle clips and clean out the corrosion that has built up underneath them. Coat with your favorite grease and reassemble with new pads/rotors. I recommend checking the pads slide freely every tire rotation or every other. HTH.

The brake fluid was just flushed out about 5 months ago.

There was a bit of corrosion on the carrier, so I will need to put on a coat of grease.

Thank you very much.

Don't just grease it. You need to remove material away. I use a file most of the time.