Uneven brake pad wear?

Rear disc brakes started making the death squeal last week, so I ordered the pads/rotors and swapped them today. Three of the four pads had about half their “meat” left - it was just one (outer pad driver side) that was worn to nothing. Slide pins were fine, calipers looked okay, retracted fine (did it by hand, didn’t need a clamp, so definitely not seized) and I get even braking on both sides. Test drive afterwards was uneventful. Firm pedal, no noise. The brakes were original to the car (2012, 120k miles) so I don’t mind replacing them even if it might have been early, just wondering what might cause that? (Am I looking at caliper replacement soon?)