Only a guess, but it could be a couple things. Possibly it is prepsring to spawn, and the bulge could be that build up. Another idea is that it may be some air bubbles are trapped inside. If this is the case, then there is nothing that can really be done.
Just out of curiosity, are these swells concentric around the mouth? Because mine do sell significantly near the mouth on a nightly basis, but it is gone by morning.