A phobia is an unconscious response to a very specific trigger that has been been learned, usually before the age of 7 years old. The brain is a fantastic learning machine with a prime directive of preserving the integrity of the mind and body system. A phobic reaction is a very intense reaction to the stimulus.
When a significant emotional event happens, the brain unconsciously scans the environment in minute detail (visual, sounds, feelings, smells and tastes). When the brain detects a stimuli that is not normally there (E.g. Bird, Spider, Bug, etc), the brain assumes that the stimuli must have caused the danger and remembers it (Association - Bird = Danger). Any time that stimuli comes back, the brain kicks off a fight, flight or freeze response to the stimuli as it assumes it is dangerous.
Phobias are not rational or logical, but rather are the result of unconscious learned behaviour that have been programmed into the mind.