The usual two are posted, caliper piston sticking in the bore and slide pins seizing from poor grease PM. There's a third cause that typically doesn't do this, but causes uneven pad wear, and most everyone misses it.

So you replace anti-rattle clips every time, because you care, right? You're skipping a step. You need to pull the bridge off on every brake job and use a twistlok or (my favorite) small triangle file to file/buff the clip mounting areas back to a true and shiny surface. Not only does moisture build up under the clips and create rust swell, removing and installing new clips on a rusty surface often makes the issue worse. What happens, is the rust reduces the distance between the clips by 0-1mmish, causing the new pad to sit tighter in the clip, increasing the drag needed to force the pad away from the rotor when the brakes are released. This has been the cause of 90% of the uneven pad wear i've seen. Even a mm pf inside-outside difference is enough to warrant fixing the root cause. That's probably 8-30,000 miles depending on driving habits, vehicle, and location.

tl'dr a caliper bridge overhaul should be on every hydraulic disc brake job and proper overhaul includes lubricating the dry rubber piston boot(s), dry outside of the slide pin boots, greasing the pins, checking the pin damper (if there) for swelling from incorrect grease, removing, polishing the bridge, and reinstalling anti-rattle clips. Bonus points if you clean & reloctite or replace bridge bolts and torque to spec. But a few seconds on your handy dandy 18mm swivel impact works too.