Why Does Your Dog Kick Backwards After Defecation?
Most dogs, after "doing their business," will step away, and kick backwards with their rear legs. It may appear, since we tend to anthropomorphize our pet dogs, that your dog is trying to cover up what it has just done. But actually, the opposite is true. Once again, when looked at from a wolf's point of view, it is essential to scent mark the wolf's territory. So contrary to covering up, your dog is imitating an action that is meant to spread its scent even farther. In the wild, the kicking motion would kick up sticks, leaves, and other debris over the feces, spreading the scent over a wider area. Scent marking is a vital way to avoid conflict with wolves from an adjacent territory. Wolves seem to have a number of techniques and practices that avoid coming into unnecessary contact with wolves not of their own pack, as these encounters can sometimes lead to a fight to the death.
(Source: Zoologist Desmond Morris, Author of "Dog Watching."