Not technically aggressive behavior, this is the instinct to remove self from a threatening situation. This is the most insecure drive and is the opposite of prey drive. Flight drive is the drive a dog will be in if the dog feels he is the prey. A dog that is fighting back is not technically in flight. Flight is total retreat. Some trainer's refer to dogs that are easily triggered into flight behavior as "nervy". If a dog is triggered into flight or is hesitant to specifically engage in certain environments, surfaces, or obstacles he may be referred to as having bad "environmental nerves". It is important to note that flight behavior is a very strong survival instinct that is present in most wild animals. Even the term "bad environmental nerves" incorrectly suggests that there is something clinically wrong with the dog". The truth is that "good environmental nerves", although desirable for most domestic dogs, is actually the result of a genetic suppression of the natural instincts of flight and hesitancy in potentially dangerous situations.
To learn more how flight drive relates to aggression read: Aggression Drives