As I recall, in several of her Pocket Morning answers, Riho has said things about wanting to get rid of her "bocchi" role. She always sounds like she's half joking, but she's said it often enough to suggest that there's some truth in it. Whether the role of loner is one that was handed to her by H!P Central Casting or one that she just fell into, I can see how she might come to really, really dislike it, if it's not a role that she chose. So maybe discontent at being stuck in the bocchi role is at least a contributing factor.
I always liked Riho, and I'm sad to see her go. I've never liked the whole "ace" concept -- it just seems like more showbiz hype, no matter who it's applied to (remember - Morning Musume started out as a pack of deuces, and that was always a point of pride), but I think that she brought a lot to the group with her singing and dancing, and also with her personality (which always struck me as being likeable and charming, even with her kind of smirky-looking smile

).
And because in the world of H!P fandom, it's never too early to speculate -- I wonder if we're going to see a 13th gen audition announcement soon. I kind of think we will...
If madness made us strong, we would all be invincible.