@ahemorrhoid
@Nik_Y
The only reason (other than cheap drama) I can come up with is Koharu seeing the writing on the wall about the Harekon system being abolished legally, and she knew it would be more complicated for Yuzu and her child to not be the one legally tied to Ryuu but felt like Ryuu either wouldn't make that choice or would choose Koharu over Yuzu so she forced his hand by dumping him first.
But then again, that all got subverted so quickly if that was the point NON was trying to make it got washed away by that whiplash lol. Yuzu and Madoka had a point, if the Harekon system was no longer legal the divorce may have been necessary, however leaving the relationship altogether was certainly not.