The cause of this a lot of the time is that you get gunge on the bolts that the caliper slides along, mainly due to the rubber seals breaking down and the grease combining with brake dust and mud.
You can normally see it happening with the wheel off and one side of the caliper will move, and the other will stay still. Only problem is, it's also one of those problems that can show up intermittently, making tracking it down hard.
If it is that, often a good clean and regrease will solve the problem.