While trying to think of examples, Shizuka Kanno suddenly came to mind. I mean, the answer to the question is clear. What happened to her? She just stopped making AV a few years ago. But I wondered, was there ever a big announcement, a retirement feature or something, or did she just quit like that.
So I stumbled upon her old blog, and the last entry might shed some light on the mystery, if we can get someone with japanese proficiency in here.
http://blog.livedoor.jp/kannoshizuka/archives/65804403.html
As always, Google Translate doesn't help much and is highly unreliable, but from what it looks she got into a dispute over a breach of contract, when the company sold uncensored versions of her movies overseas which she didn't agree to and didn't want or something like that. I Hope someone can clarify this.