"What is there not to bully?"
-Hyunjin.

He doesn't hate you, he just really doesn't like like you LMAO. I mean- what is there to like?
Why he does it is both unclear and blindingly simple, the specific thing you did? No idea. The reason he does it? He doesn't like you.
Duh.

But otherwise I mean like- you're in Stray Kids, and everyone else likes you or at least tolerates you, it's just him that lowkie tries to hurt your feelings just to be petty.

Have fun...