They aren't fighting, per se, but playing and figuring out how to be adult chickens. Just like puppies wrestle, kittens pounce and colts fight and race.
They are also figuring out their pecking order. That's one of the most important facets of chicken life to a chicken.
They will calm down. They may always peck each other a bit sometimes, as they remind lower-order flockmates to stay in their place.
Roosters don't fight all the time, either. When you have more than one, they may spar a bit at first to figure out who is stronger, but then they will pretty much ignore each other as long as you have enough hens to go around and enough space to stay out of each others' way.