These drummer are possibly seeking cleaning by unseen hosts that are stationed on the jetty piles or the associated common kelp. I can't prove this, but again their behaviour is suggestive( they were not feeding,being outgoing tide, and they loitered persistently around the mid to upper piles,and in top L corner of 1st image you can just see a drummer's head with open mouth.That suggests the individual is more desperate than average to achieve parasite removal,based on what little is known about temperate fish cleaning.)