[Z_Fanatic]

I would guess that the metal spring that pushes both pads together were ill-fitted or got loose. So one probably rubbed pretty hard, and the other went out of line and barely gliding on the rotor. It could be also that caliper wasn't set correctly, my bike once had an issue where it screeched every time I backed up the bike, but I don't think you have that problem. If the outerpad even has the slightest decay, it's best to replace both of them and make sure they both sit on the disc firmly. Also check if the rotor is warped or not.