It's difficult to know what sparks a fear response in your dog. That is, sometimes it is difficult. A reaction to fear can be immediate but can also be delayed. It's the delayed response that gets us.
We sometimes hear the phrase "out of the blue" regarding a dog's behavior. "She lunged out of the blue", "He nipped out of the blue", "She growled out of the blue", "Out of the blue he suddenly wouldn't (fill in the blank)".
When we look back and analyze events in the dog's life there are often bread crumbs that lead us to an event or series of events that led right up to the "out of the blue" behavior.
Because it takes some detective work to get to the root of a problem, it behooves us to avoid the urge to scold or punish when the undesirable behavior occurs. If the dog is reacting to something that has caused her concern and then her person is upset on top of the scary thing, her belief that the thing is indeed as bad as she feared is reinforced.
The rabbit hole might look like this:
•There's a thing that is troubling to Dog
•Dog thought the thing was bad so ignored it or tried to keep distance
•The thing didn't go away so Dog did something to communicate dislike or unease
•Thing still didn't go away so Dog did something more dramatic to make it go away and person yelled, jerked leash, hit, etc
•Dog concludes that, sure enough, the thing really IS BAD
The rabbit hole can be turned around:
•Dog thought the thing was bad so ignored or tried to keep distance
•The thing didn't go away so Dog did something to communicate dislike or unease
•Person took Dog away from the thing and spoke kindly
•Next time the thing was around, person let Dog stay as far away as he wanted and gave something valuable to Dog when thing was in the environment
•Next time the thing was around, person had more of what Dog loves and talked kindly when the thing was in the environment.
•Next time the thing was around, the things Dog likes were around as well. Kind words, treats and pats also go with the thing
•Maybe the thing isn't so bad.
Of course not everything is as simple and quick as it looks when reading this simplistic description of the rabbit hole or behavior loop. However, the principle still stands that our negative reaction, paired with something that is troubling only reinforces the dog's fear or wariness.