Legend of the Phatasour is my favorite Scooby Doo movie, for several reasons.
But that’s not the purpose of this post.
In the movie, Velma recounts a typical evening that ended up making Shaggy’s extreme fear response cause him to scream for three hours straight without taking a breath. The recap of the mystery that caused Shaggy’s breakdown ended with Shaggy locking himself in a wardrobe, screaming. Throughout the rest of the movie, when Shaggy started to get scared, he’d freeze with a strange expression on his face, quiver, whimper, and hide.
This clearly demonstrates that Shaggy has PTSD. On a “typical evening” he was suddenly triggered and could not break free from a deep sense of absolute terror. Shaggy’s hiding in the wardrobe could be reflective of the behavior that saved him during his trauma experience - hiding from the life threatening entity. Then, after a major flashback, many other triggers caused Shaggy to dissociate and revert back to the coping mechanism that made him feel safe.