My dog is afraid of plastic grocery bags. I've often thought he must have had a traumatic experience with one as a pup but, truth be told, my dog is just a big old wimp. He's afraid of anything that might move suddenly even though he's the one making it move. So if he starts playing with something that's lighter than his tennis balls he jumps. The poor little guy is deathly afraid of cat toys. He's afraid of the sound labels make when they're torn off of something. He runs everytime I open a new cd or dvd. On the Fourth of July and New Years, he's an absolute basket case.
I am eternally grateful, however, that he runs and hides in any small crack or crevice instead of barking like a crazy dog. Now, get him around other dogs and he doesn't act afraid. He doesn't socialize but at least he doesn't cower...cuz he's macho like that.