What could make your caliper stick?

Other than a bad caliper, what could make your caliper stick. I had sticking caliper, replaced it, the rotors and pads, bled system and it worked fine, now it seems like it could be sticking again.

Had sticking caliper that wore pads to nothing on drivers side while all others remained good. Replace calipers, pads and rotors up front, bled the system and although the drivers side didn't push to the floor as good as the passengers side while bleeding, they bled out and it worked good for about a week. Now, it feels like the drivers side is sticking again, could it had been something else to begin with?