It could be because its the latest memember of your tank. Remember fish are teratorial.
For example my Yellow Tang was the first to enter my tank and still is the biggest.
When i added my latest fish (a regal tang) the yellow tang bullyed him by nudging him, swimming into him and slapping him with his fins.
This is just to certifiy that hes the boss, it settled now and they follow each other around :)
try waiting it out