Probably stating the obvious, but for some does it may be a protective thing - like when they think they get a whiff of something wrong and start investigating till they find you to protect their harem.
For some bucks it may sound like a buck hitting a rub. I know this year in KS I could hear a couple bucks in thick stuff making all kinds of thrashing racket - sounded like one knocked down a tree - talk about a lil frustration! So if it isn't heavily hunted, a curious buck may check out who is in his territory. Some guys thrash the bushes/tree when they rattle - not a bad idea.