JK herself said, "being Robert Galbraith has been such a liberating experience. It has been wonderful to publish without the hype or expectation." I hear you, sister. I too prefer publishing without knowing that millions of people are camped outside bookstores eager to get their hands on my newest novel.
Oh, wait, not so much.
Oh, wait, not so much.
But actually, the pressure on her must have been tremendous, and the fact that The Casual Vacancy was not as well received as had been expected (because, seriously, how do you follow Harry Potter?) might have left her feeling a little fed up with the whole business. In those circumstances, it's hardly surprising she wanted to dispense with all the media frenzy and just be a normal author. Maybe she wanted to see how well her book sold and how well it was received without wondering how much those reviews or sales figures were influenced by her name being on the cover.
Well, it had some excellent reviews from people who are doubtless now feeling vindicated, and sold 1,500 copies in hardback, which some commentators have said is "disappointing" but personally I think is excellent. I know I'd have been thrilled with that anyway. I've only twice sold that many copies in paperback. So good for her, and I'm sorry she was outed earlier than she had hoped. (Particularly sorry because some of the newest reviews are now focussing on Harry Potter and her little stunt, rather than the book itself.)
She's not the only author to pull this trick. Madeleine Wickham was a successful author who wrote a book in a slightly different genre and sent it anonymously to her agent. The agent accepted, all the while thinking, "If this didn't say Sophie Kinsella on the front I could swear it was written by Maddy." Sophie Kinsella is now more successful than Madeleine Wickham.
Steven King also writes under the name Richard Bachman. Ruth Rendell also writes as Barbara Vine. In some cases authors choose to write under a pseudonym when they switch genres, but not always.
I've written in many genres (gentle women's fiction, historical, comedy, romance, thriller, religious and sci-fi) but always under my own name. Mostly because I can't afford to set up different websites for all my extra identities. But also because I'm saving the pseudonym for when I want to put something out without having to face all the hype and expectation.