Okay, I'm getting a better idea that Keiko is going away related to her job. Probably some temporary transfer where she'll be away for some extended period of time. And since it's her apartment that Ran can't afford alone, they have to vacate entirely. I'm not feeling the depression so bad since this is pretty clearly a separation that allows both to sort their feelings out, although Ran's got a pretty firm grasp on hers.
idk, it might not work out but if I were writing a yuri story of this nature, this is probably how I'd handle it.