Hi there! Your answer is: PTSD.
Let's talk about how PTSD is formed, though. PTSD stands for Post Traumatic Stress Disorder. It forms when something so traumatic happens to someone, anything similar to that freaks them out(in a way, it triggers fight or flight)
Here's an example. If your father hit you as a child, his arm would raise to swing. That person later in life may develope PTSD from this, and whenever someone raises their arm in a abrupt or quick manner, it can scare them by bringing back unwanted memories.
While this is upsetting, it's part of a common example.
By the way; PTSD is nothing to joke about. It's a thing a lot of people suffer from.
Good luck!