Usually, fish yawning occurs as a display of territoriality or to attract females. If your fish is yawning at nothing, it's for the same reason people yawn...an abundance of CO2. If your fish are yawning all the time, do a water change and boost aeration in the tank.
All backboned animals yawn and though there are a lot of theorys there is no real answer as to why. If your fish aren't doing in constantly I wouldn't worry, it's normal for them to do it a few times a day, just like us.