Because the animal became injured. If he just thought he'd just shoe it away he might not care what she thought about him footing her dog. No one wants you kicking their dog, but a woman is much less likely to become physically aggressive if you do so. Calling the police was mea culpa when he realized how bad what he'd done was.
But seriously, who's kicking a dog for fun, even just to get it out of his way and not because he thought it would attack him? I certainly wouldn't call the police afterwards as the dog owner would have a case against me.