Publishers seem to prefer to keep their authors focused on a particular genre after they've achieved at least some initial success. No doubt there are business and marketing principles at work, and there are undoubtedly payoffs for both parties, but it might also be like fitting the author with a pair of horse blinders (remember those side flap goggles worn by the horse pulling the junkman's cart, to keep the horse's attention on the road ahead?).
The publisher may feel it has money invested in the author's name--the brand--and has hopes of building a faithful, ever larger consumer base for the brand. Our author meanwhile may be pleased by the past commercial success, but he's an artist for god's sake and may want to give free rein to new creative energies. So what if a venture into the new genre doesn't sell as well? Well, life is hard, money is tight, shareholders have expectations, and authors might be a little crazy. Still, if an author has a day job to meet subsistence needs, riding a new bull at the rodeo might be exhilarating.
Famous authors are more likely to get a nod from their publishers when submitting cross-genre work. Some whom I have read with good crossover adult, young adult, and middle grade novels within their individual collections include Louise Erdrich, Carl Hiaasen, and Neil Gaiman, to name just a few. So it can be, and is, done. It's just less of a financial risk for the publisher, or career risk for the author, if the author already has a following.
Of course it's also less of a risk if the author is still inhabiting the same moral and physical universe of his other genres. Neil Gaiman might not reverberate in romance genre as well as in his more typical fantasy genre. It could be interesting to see what happens though.
Another way to potentially upset your hardworking publisher is to run your next piece of work past him with a pseudonym on it. "Some famous authors publish under pseudonyms so that they can get a fresh reading of their work," says an article in the NY Times (2/23/2012). "In 1987 Joyce Carol Oates released a book under the name Rosamond Smith but apologized and swore off pseudonyms when her publisher discovered what she had done." Apparently they didn't think it was a very good decision in her case, but authors might resort to using pseudonyms for various reasons. In earlier times women authors sometimes adopted men's names in hopes of being taken more seriously as writers. Joanne Kathleen Rowling took the neutral gender J. K. Rowling in hopes of better attracting more boy readers.
The same Times article discusses an author, Patricia O'Brien, who had published several books including a novel, but whose most recent novel had been submitted to 13 publishers by her agent without finding a home. An Internet check on BookScan showed it had sold only 4000 copies, which was considered a flop. However, her agent, who had a lot of confidence in the book, said "I realized that the book was not being judged on its merits. It was being judged on how many books she has sold. I needed somebody who couldn't look on BookScan." When the book reached another publisher under Ms. O'Brien's new pseudonym, Kate Alcott, there were no adverse digital footprints found on Internet searches, and it received an enthusiastic reading, and was accepted. In time Ms. O'Brien came clean with the publisher, everyone remained friends, and the same publisher later bought another novel from Ms. O'Brien. A fortuitous outcome in this case.