Hackles; is the hair on your dogs back. It goes from right behind their ears all the way to the and on the tail. Usually just the hair on the shoulders goes up easily with the neck and tail end being reserved for really big things. The hair on our dogs back goes up in times of alertness. It could be caused by fear or dominance. Fearful dogs approaching another dog will raise their hair as will an aggressive dog.
Situations like would be intruders can raise a dogs hackles. I think lastnight my guys hair would have been up. I know Jessie's was but I never know if the poodles hair is up or not. Although they both shook when they came if after fearlessly advanced on the invisible intruder in the backyard which is a good sign that it was up. When Jessie is really ticked off it goes from her neck all the way down onto her tail. She is very fierce looking. Jessie's hair even goes up when she is having an intense dream; raising shackles is an involuntary behavior.
It is very similar to turning red in the face. Some people have a very quick trigger to the red face situation as some dogs have a quick hair up trigger. But to really know what is going on with raised hair you must look at the whole picture, not only the hair. What are the ears, tail, head and body saying with the raised hair?
Puppies normally have hair triggers so to speak. Their hair goes up easily because they are not yet completely confident. But no matter what the cause the result is a dog who is not in a calm relaxed state.