.
"The novel at volume 4 (I believe that's the latest one) gets even more yurilicious as things go on. After the whole ordeal with Reina and Taki-sensei going not the way she expected, she starts drifting more and more toward Kumiko to the point that it seems she's legitimately falling for Kumiko, and Kumiko starts to reciprocate.
Now as far as I'm aware, Kumiko doesn't fall for Shuuichi. it's more of a "agrees to date" kind of thing (volume 3). So my best guess is that Hazuki pushes for it hard, and for some reason Kumiko feels like she has to do so to respect her friend (and it probably happened around the same time she heard about Reina 'liking' Taki-sensei so she probably subconsciously rejected the idea of her as an option). Based on what we've seen from this novel's writing, I bet after not too long Kumiko will pull away from Shuuichi.