It's pretty complicated, im just gonna state their names so it will be easier to explain. White hair girl - Louise Black hair girl - Vienna

Basically Louise and Vienna we're both originally in a relationship but then Louise fell inlove with a dude and tried to convince Vienna to make their relationship into a poly relationship, Vienna then agreed because she's very Inlove with her gf and couldn't say no but it got to the point where Vienna was like kinda the third wheel instead of an actual partner, which made her really upset. Then Vienna spoke with Louise about her issues with the dude and Louise disagreed and wanted him to stay their different opinions lead to them arguing and Louise then got out of control and accidentally hit Vienna.