So, suppose you are a bestselling author, and not just a bestselling author but a blockbuster author whose name on a book creates explosive sales. You are, however, essentially typecast as an author of a certain kind of book, the kind that made you unbelievably famous, and whenever you try to branch out, people buy your books because of your name, but you are savaged by critics and reviewers. So what do you do? You submit a work under a pseudonym, in a different genre. It takes some time to get it accepted, because it doesn't come with that magic name, but when it finally is, it turns out to be a modest success. It doesn't have the explosive sales that you could get with your real name on the cover, but the sales are solid, being exactly what you'd expect of a book successful in an ordinary way in that genre, and, what is more, the reviews are actually quite good. Your plan is working: you are showing that you have actual writing chops that people will find at least respectable in their own right, that it is not merely the fluke of having become unbelievably famous with your first books, and that much of that critical savaging was nothing more than people trying to make a name as a critic at the expense of an author of name, rather than a genuine indictment of your abilities. But it eventually leaks out that Robert Galbraith, up and coming mystery author with a very promising first novel in the field and another book already sold, is J. K. Rowling. (It's happened before; Stephen King famously published for years under the name Richard Bachmann, and was also a mid-level success as such. Once it came out that Bachmann was King, Bachmann's sales exploded, because King as King had a massively larger market, but Bachmann's sales were already brisk and increasing. The Bachmann name, like the Galbraith name for Rowling, gave King a chance to play around a bit with new things without any of the hype or expectation.)
I've always thought Rowling, despite the defects of some of her work (they are, after all, early novels, however famous they became, and after the first, novels published under extraordinary pressure), was not quite given a fair shake as an author in some quarters, but this whole thing makes me like her even more: there is something very heartening about a blockbuster novelist deliberately not using her name just so she can try her hand at doing some competent work in quiet.