Another reason is that people write books in a certain genre, become famous for those, and then want to try something new but the publishers won't let them work under that name. Nora Roberts, for example. She is a renowned romance author, but when she wanted to start her murder mystery series, her publishers wouldn't let her publish under Nora Roberts. She used the name J.D. Robb, and when they realized that those books were selling VERY well, they started publishing them as "Nora Roberts writing as J.D. Robb" to receive more attention.