Maybe she's just tired of public life, like dozens of idols before her. Become a regular hick girl again and not a target.
It's harder than you realize.
She was too traumatized by the appearance where that American rocker groped her the whole time.
"Nuh uh... don't wanna be a celebrity any more..."
Anyway, as I found out about this before I went to bed last night and discussed it with a few people then, it's had a bit of time to sink in. Honestly? Somehow I don't care. My best memories with Koharu were always related to Kirari - Balalaika, Happy Happy Sunday!, Kira*Pika/Milky Way etc. I guess it just never felt like she really fit with Morning Musume. During Kirari it was that voice they made her have. Recently she'd seemed to be developing something else, but as others have said, she still seemed a bit separate. She was just the lonely nanaki member. Ai is leader, and has her Gaki subleader and gokkie compadre. KameShige have always been best friends, and although Reina was her own lone yankii, she still had her rokkie mates. Of course the others have their hachiki community, and are still also developing their images.
Somehow I always thought Koharu would be next, since at this point I really didn't see how it could be Ai-chan, much less any of the others.
As to reasons... It seems to me from the comments that they're just mentioning modeling as something Koharu's wanted to do, but almost as if they just mention it to be able to give some indication of what she might do from now on. It sounded to me that it was more a case of "finding what fits her best". It doesn't sound like she really graduated to specifically do something else. And she's still young, so has time to figure out what that might be.
What was the real reason for the sudden decision for her to leave? We might never know for sure, but it seems apparent there's
something behind it. And even though I'm not surprised, I liked Koharu well enough as I mentioned for example her Kirari days, and hope she finds happiness in whatever she does. It's been a while, but graduations I've learned long ago are a way of life as an H!P fan. I feel sorry for all of you who have only come to be fans in the last 2 or 3 years and may not have experienced one yet.
So I guess we're getting set up for a kyuuki audition as usual once winter arrives? Timing is definitely appropriate for that. I'm curious to see how they'll promote it.