Why do dogs kick their back legs after pooping and peeing?

Some do and some don't. I was dog sitting last weekend for a bit and some of the time that dog did it and some of the time he didn't. My own dog years ago didn't do it, as far as I can remember.

I can think of two possible reasons, firstly to disperse scent marking and secondly to move faeces away from an area the dog is living/sleeping in. (Although coprophagia is a problem among dogs.)