Responses
I'm guessing trauma from his dad abusing him.
It’s shown more early on, it’s because of his dad
Like what the others said, it’s PTSD from when he was a child. His dad would grab the back of his neck from behind and threaten him. He once even held a knife (looked like a scalpel or box cutter or something) to his face.
Did i forget something? Why was yujin so startled and panicked when that bastard grabbed the back of his neck??