Hey my turtle does the same thing a while ago I read it might be from a nutrition problem if you don't feed it some edible plants, goldfish, and get your turtle a calcium cuddle bone. Hope it helps :)
It's a sign of dominance. It's a territorial threat to other turtles. Mine recently began doing the same thing toward my other two. It's an early warning sign...may be time to seperate him from your others if there is more than one.
Lol he is probably basking or when swimming they do that. No need to be concerned.