It is fighting behavior. I had a school of green tiger barbs, five of them, and they started to do this dance. One by one the next day after the dance I'd find a dead barb. The most dominate fish killed off the other four of the school. That dominate fish is still alive today (four years later) but I have not put any more barbs in the tank because he will probably kill them all again. He roundly gets his arse kicked by the rainbow shark now, and is definately the omega fish of the tank<G>. See, what goes around comes around.
~~Colesea