My only guess is that the faucet has a leak. Frost free hose bibbs can freeze during the winter (especially if a hose was left attached to them all winter). But because of their design, you wouldn't know that it froze until you go to turn it on in spring. Usually, it is obvious that there is a rupture in the shaft of the hose bibb because it leaks water like crazy when you turn it on. But perhaps yours is set up in such a way that masks the leak. Does the hose bibb go through the exterior wall into the crawl space? If so, check the crawl space to see if there is a big puddle of water there. If there is, then you'll need to get that hose bibb replaced.