According to the AKC Canine Health Foundation, fear is the most common cause of dog aggression, and not dominance as most people tend to think.
Fear in dogs is mostly created by dominance-based training. The result is rush and uncontrolled responses to social and interactive situations.
Fear aggression is most common in younger dogs below 3 years of age and is portrayed as a form of defense.
Dogs with fear aggression may be afraid of certain categories of people or animals and can easily generalize the fear towards similar people/animals or situations where the threat has been perceived.
Dominance aggression is a dog’s way of saying it is the alpha in charge of the situation. If a dog feels that its position of dominance is threatened, it will put forth aggressive behavior such as staring, standing over, growling, and biting.
Possessive aggression is manifested when people or other animals touch or go near objects, people, or places that the dog perceives as its territory. A dog may bite a neighbor who passes by the home because that is its territory.
Other causes associated with sudden dog aggression include frustrations, redirected anger, and age or sex-related changes among others.