Dominance aggression and protective aggression are the number 1 and number 2 causes of treatment by behaviorists. From the description behavior, it might be possible to rule out actual interdog aggression. The dog is challenged by a stare or a bump or body block, and then each dog behaves in reaction to what the other dog did. A characteristic of interdog aggression is that aggressive intention is not displayed to other animals. The dog may live amicably with cats and horses.
Protective aggression is stimulated by sudden movements.
This type of aggression is the type which is looked for at the 8 week puppy test. For dominance aggression, in contrast to protective aggression, there is more growling, snarling, biting, and staring. But the worse dominant aggressive dogs I have ever seen have been toy poodles and shih tzus - their behavior is more likely to be seen as innocent and owner tolerant. You can see how these signs are tolerated in smaller dogs. And then there are those dogs where all the signs were there.