Dog's and cats, have scent glands in their paws. When a dog kicks after going potty they are putting their scent on the area with their paws and marking their spot, to let other dogs know it is their "territory".
Dogs are marking their territory when they kick after they pee. Dogs have sweat glands in their paws, when they go pee they will scrap their paws along the ground to mark their territory. They do this in hopes other dogs will be lured into their territory. They also do this with poop.