So ummm

here's the reading order if you're interested more in the characters and their past!!

0. Aazami - past Yumi x Kan
1. Sabita Yoru Demo Koi Wa Sasayaku -Yumi meets Mayama and breaks up with Kan chan
2. Renai-rubi no Tadashii Furikata (ch3.1 - 3.5) - Kan meets Shuuna and turns uke
- Narrow (Place collection) - dj Kan x Shuuna smutty sidestory
3. Hadakeru Kaibutsu - continuation Kan x Shuuna (Shuuna's POV as far, timeline parallel to no.2 for ch1)

but to answer your question, the one shot in Aazami explains why Kan chan is like that