I will cherish this video forever. You could put a lil prep h on his bum and see if he keeps doing it. That would tell you if hes itchy or just...weird.
How old is he? I would say that is misplaced matting practice. Basically, he's reliving a bit of tension, not sure how else to put it. Does he have access to any hens? If you slide your hand under his rear end when he does this you can see if it is covered with spray.
Just because it's natural doesnt mean its pretty. Its natural for a baby koala to eat its mother's feces to conform to the diet of eucalyptus. Truly gross to watch.
While not as entertaining as the OP's video this is a young cockerel doing much the same but with my boot.