It doesn't matter how your dog became fearful, it's how you deal with it that counts.
Dogs generalise fear, anxiety, even aggression much faster than positive emotions. A fear inducing thunder-clap quickly 'generalises' to a similar noise like back-firing cars, resulting in 'flight' or 'fight' mode.
For environmentally sensitive dogs use games to rehabilitate, visit strange places and play games for two minutes. It's often a slow process, your dog's attitude, confidence levels will dictate progression.
Avoid patting to reassure as you are praising and increasing the fear/anxiety. Praise only for an action, never an emotion.
The 'generalise' fear/anxiety concept can rapidly transcend into body parts, physical items, locations, vehicles or situations and is problematic when it turns into aggression.
If there is a problem don't avoid it. Your dog won't recover without help, the solution may be a phone call away.
- Daniel Rodger, Good Dag Bad Dog.