If this happens after heavy braking, then it could be that the fluid is contaminated and starts to boil in the caliper. It is rare, but it is a possibility. If the brake fluid is DOT3, then it is hygroscopic in nature, and if it has absorbed a lot of water over time, then the boiling point will be reduced. I don't believe that calipers need to be replaced in pairs, but that seems to be the current mode of thinking. I believe that the logic is that if one goes bad, that the other isn't far behind. When doing brake service, the best thing is to do it well, because unlike bad engine maintenance that will leave you stranded on the side of the road, bad brakes might leave you in the middle of a tree... Do it once..... do it right.