It's a bit of pecking order, and a bit of play. All of my pullets from the last 2 batches have gone through this sort of behavior with one another when they get let out for some free range time - it's like once they have the room to horse around they'll taking advantage of it (just like how my dogs want to tussle outside, not inside). They'll probably continue to do it routinely until they're older and less hyper and more set with their pecking order.