Our own fight or flight response was originally useful for getting us out of danger; triggering that adrenaline rush is why people like scary stuff in the first place.
Dragons are basically lizards in the first place. These ones don't even have wings, and I can't think of any examples of them breathing fire. Even as an apex predator, detaching their tail could still save their life. In this relatively peaceful setting, dragons no longer face danger in their daily lives, and so would seek out their adrenaline rush the same way humans do.
All this is pretty pointless, though. The author probably just thought it would be funny for a dragon to lose part of its tail while watching a scary movie.