Here is a great article that covers some common triggers for dog aggression. Aggression can simply come from the way you are picking up your dog, trimming their nails, bathing them or even just brushing them. This is usually do to the stress it induces. This also goes for most types of check ups or almost any handling by strangers.
Here are some common triggers for dog aggression and what to look out for:
Puppies: Maternal aggression is common in all species. Biologically, the point of all life is to pass on genes through reproduction. Because this instinct is strong and inherent in all animals, mothers are extremely likely to be very protective of their litters. Even a dam that is usually friendly may consider strangers to be a threat to her litter and display emotional signals which are intended to inhibit further approach.
Territory Invasion: Many dogs think guarding their home and property is a very important job. Territoriality is an extension of resource guarding, when the entire home and property become a valuable resource which is to be guarded from intruders at any cost.
Resource Guarding: Resource guarding is natural behavior. Dogs that resource guard will view approach by other dogs and/or humans as a threat to what they perceive to be valuable – be it the home property, the owner, a meal or a toy, or a preferred sleeping space.
Other Dogs: Aggression toward other dogs may have a variety of manifestations and causes:1. Intersex aggression – Intersex aggression is aggression toward dogs of the same sex. This tends to be most common in dogs that are sexually intact and is generally resource guarding for reproductive advantage.
2. Type-specific aggression – Type-specific aggression can occur when a dog has a socialization deficit with dogs of a particular body type (large, black dogs for instance) or a history of negative experiences with a dog of particular body type.
3. Behavior-specific aggression – Dogs, like people, cannot be expected to indefinitely tolerate even the rudest behavior of conspecifics (other dogs). Many dogs will not hesitate to use their voices, body, and/or teeth to tell a rude dog to “back off!”
Movement: Because dogs are predators, they are hard-wired to chase after and bite at things that move quickly and/or unpredictably. Animals which move quickly (squirrels, birds, cats, etc.) are frequent triggers. Human triggers for motion reactivity include biking, jogging, skateboarding, or moving automobiles.
Frustration: Frustration is another common cause of dog aggression. Frustration creates stress, which contributes to aggression. Frustration aggression often forms in relation to barriers including leashes or fences. The dog may want to check out a person or dog on the other side of the fence but becomes frustrated because he cannot. He may redirect his aggression toward a familiar human or animal as a result. Frustration aggression may also occur in relation to extinction, where reinforcement is removed for a behavior that has been previously continuously reinforced. If barking always worked to get attention but suddenly the owner begins ignoring the barking, the dog may experiment to find out if nipping is a more effective way of getting attention.
Dogs sometimes have tension towards certain types of people due to facial hair, size, color or any other different types of people they have not yet experienced.
Did your dog struggle with any certain types of people or children at first?