Heuerfan, the cause can be a number of things and difficult to pinpoint, but here's a few things to consider.
Stress - persistently being chased by other fish and not having enough rockwork to retreat into. Fluctuation in water parameters can also stress out a fish. Over crowding can also be an issue even if there isn't a "bully" in the tank.
New Inhabitants - New Fish, corals or even rocks can bring it into a system.
Poor skimming/no UV Sterilizer - both worth their weight in gold as far as keeping a healthy tank in my opinion. Good luck, I hope you get it under control. Get yourself a Cleaner wrasse if you dont' have one, but two important things to consider when buying one. First watch and make sure you pick one that's picking at other fish so you know he'll do his job. Next ask your LFS to feed mysis or brine and make sure he/she takes it, because they'll need supplemental diets besides picking at your fish. I hope this helps.