What the... Why??? I demand another post, where they do this and that... you know what, in the same state, with even more rigor and intensity!
(But still... Why???)
Because artist wanted to draw Kotori and Umi punching the shit out of their beautiful face?
Also considering Kotori's position, she's basicaly a one move from throwing a very painful for Umi punch. A full swing into Umi's face with pulling her head forward to maximize the power will definitively end it.
Kotori can be very emotional at times, and it looks like Umi struck a nerve with her. But where were the other girls to break this up?
Is the Idol Research Club secretly Fight Club?
I can imagine this situation as Kotori's very exaggerated answer for when Umi slapped Honoka. You know, Kotori was always Honoka's yes-man so seeing something like that could lead to fight.
Also, seeing scoring of this image probably we will get obligatory shitty reason flag.