Phobias take many forms in both people and cats; with these fears often routed in traumatic events. We develop these fears as a survival mechanism. It is likely that your cat was startled, and thus developed a fear response to a past plastic-bag incident such as getting a bag caught around his neck as a kitten. This incident taught him that plastic bags are scary and thus should be avoided. Every time your kitty hears and sees that scary plastic bag shaking at him, it reaffirms they are scary objects best avoided.