It wasn't until DSM V that PTSD was removed from an anxiety disorder, because it's not only that. Our lives suck: our brains were trained on cues in unpredictable homes/situations (most people who get PTSD from a Type I event were primed to get it from their chaotic social environment). So the brains develop or change differently and just go haywire and hopeless over what is usually benign stimuli once we are out of those situations.