If after taking the dog to the vet and everything checks out okay health wise (no abdominal pain, ear pain, tail pain, etc) and the behavior still exists, I would say it is a dominance aggression issue.

In order to understand why your dog is acting "dominant," it’s important to know some things about canine social systems. Animals who live in social groups, including domestic dogs and wolves, establish a social structure called a dominance hierarchy within their group. This hierarchy serves to maintain order, reduce conflict and promote cooperation among group members. A position within the dominance hierarchy is established by each member of the group, based on the outcomes of interactions between themselves and the other pack members. The more dominant animals can control access to valued items such as food, den sites and mates. For domestic dogs, valued items might be food, toys, sleeping or resting places, as well as attention from their owner.

In order for your home to be a safe and happy place for pets and people, it’s best that the humans in the household assume the highest positions in the dominance hierarchy. Most dogs assume a neutral or submissive role toward people, but some dogs will challenge their owners for dominance. A dominant dog may stare, bark, growl, snap or even bite when you give him a command or ask him to give up a toy, treat or resting place. Sometimes even hugging, petting or grooming can be interpreted as gestures of dominance and, therefore, provoke a growl or snap because of the similarity of these actions to behaviors that are displayed by dominant dogs. Nevertheless, a dominant dog may still be very affectionate and may even solicit petting and attention from you.

From your dog’s point of view, children, too, have a place in the dominance hierarchy.Because children are smaller and get down on the dog’s level to play, dogs often consider them to be playmates, rather than superiors. Small children and dogs should not be left alone together without adult supervision. Older children should be taught how to play and interact appropriately and safely with dogs; however, no child should be left alone with a dog who has displayed signs of aggression.

If you recognize the beginning signs of dominance aggression in your dog, you should immediately consult an animal behavior specialist. No physical punishment should be used. Getting physical with a dominant dog may cause the dog to intensify his aggression, posing the risk of injury to you. With a dog that has shown signs of dominance aggression, you should always take precautions to ensure the safety of your family and others who may encounter your dog.

Dominance aggression problems are unlikely to go away without your taking steps to resolve them. Treatment of dominance aggression problems should always be supervised by an animal behavior specialist, since dominant aggressive dogs can be potentially dangerous.

I hope this information helps.