Isn't already because NK is getting a mix of feelings around Seungho besides the fear?
He was always afraid of Seungho and there has been more violent, dangerous situations like when Seungho had the sword with him or after NK run away and Seungho dragged him for punishment.
Only now the author is adding the badum and it happens as soon as NK is around Seungho, like when he get in the room ...Some say that it is the medicine he is taking but it's weird that only Seungho prompts the reaction.
I think that at this point is a mix of fear and attraction, even more that Seungho being restrained and showing care must be really confusing to him.
I highly doubt it's because of fear...
Yes! This!
Nakyum badump badumping because of fear breaks my heart. I hope you badump badump for a different reason soon. I wish for your happiness and Seungho's softness.