Most likely a sticking caliper, not the piston in the caliper, but rather the caliper slide pins. I'll bet that you find the caliper lside pins not well lubricated on that side. This does not usually require replacing the caliper, but just making sure that the caliper is properly lubricated and the pads properly installed. You should change the pads when you do this, and probably machine the rotors as well. It is seldom anything more than this.