Even though she felt rejected by Kurose's friend (Touya) before, he wasn't a threat because he showed he genuinely wanted to be her friend.
But IN THIS SITUATION... the girl who got her into trouble is the one who's approaching the boy she likes, that's definitely threatening for her, hahaha. I bet Koyuki just wants to be better friends with Haimiya, but even so, Haimiya is going in with the full intention of knocking her out because she thinks she's coming on to the boy she's in love with, hahaha 🤣