Did you know: Dogs kick after going to the bathroom because they are using scent glands on their paws to further mark their territory. Dogs have glands on their feet which are designed for secreting pheromones, and the backward kicking motion is a way of releasing those pheromones. They just want to make sure that the next pup who stops by, knows exactly who was there first!
This habit or behavior stemmed from wild dogs. It was a way for them to let other dogs know about their presence, size, and strength.
Source (Image): German Shepherd Dog Rescue