Ah like he won't stop staring at her which Senju assumes is a challenge of some kind? Maybe she thinks he's underestimating her because she's a girl, so she decides to solve this problem like her brothers taught her to, with a fight. Chifuyu probably tries to back out of it/calm the tension down because he's nervous doesn't want to fight a girl. But this only aggravates Senju more, who sees this as more of an insult.

https://twitter.com/fishytaiyaki/status/1480506553958469632?s=21

(Link is a pic of Senju and Chifuyu in the old thousand winter's uniform)

Imagine their ship name be two thousand tho... I know I'll be down for that 😂

(Disclaimer: I don't speak Japanese so idk the actual meaning of 千千, sorry about that)