1) Puppies not socialized or handled by breeders. Usually these guys recover very well, but they may present as very fearful at the beginning.
2) Adult dogs that have grown up without proper socialization and exposure to the outside world.
3) Puppies that have experienced a traumatic or very scary experience during the fear stage.
4) Adult dogs that experienced Number 3 during the puppy fear stage, but their owners did not realize it or did not work with their dog in the meantime. Often owners might think something like this will pass, but for many dogs it just gets worse unless they are helped along the way.
5) Traumatic experience as an adult dog. (As an example my first dog got to experience a microburst at night with us one evening. All our screens on the windows blew in, and the power was instantly out. After that, Jazzabelle was very afraid of any thunder and wind storms, which we helped her with later.)
6) Incident that may have happened that you did not see happen.
7) It could be that a medical reason, like loss of hearing, is causing them some distress. In case of sudden behavioral change, you should make an appointment with your vet.
8) So many more possibilities may exist that caused this.
9) You may never know the cause.
Bottom line is that you have to move your dog forward. They may not fully recover, but you can greatly improve their lives and their thoughts on the trigger (thing that scares them).