I think it's because he wasn't expecting it at all, maybe he saw him as a safe haven or something and when the hyung hit him, he got shocked and it broke him
/spoiler/ because it basically accumulated from the ongoing abuse Sooyoung had experienced. As a kid he was abused by his alchoholic dad and Sooyoung was in an abusive relationship with that hyung.
Everyone around Sooyoung seems to be angry at him including that hyung. When theyre together and having intimate moment, that hyung always intimidate him and being a manipulative asshole so Sooyoung then will give in and consented to the sex (e/though he said no to him). Sooyoung loves him and endure it and perceived it as "at least Hyung still love me"
So when he got hit like that (repeatedly) the emotional abuse he experienced before combine with the physical abuse he received in this incident, crushed him. It was a breaking point for Sooyoung.
Thankk you that really explained everything
Why is he so scared of hyung he said that’s what’s the first time he ever hit him so why is he so terrified of him