You are a man who strives to write romance.
Or you are a woman who has an itching to write thrillers or horror.
Let's say you've written multiple fantasy novels and now wants to branch out into horror or thriller novels.
Your name could be boring.
Or too generic.
Or too difficult to spell or pronounce.
You don't want your readers confused when you suddenly write in another genre.
You've written nonfiction, have even won awards for your work, and now want to write genre fiction (i.e. John Sandford).
Or you could do the old Stephen King/Richard Bachman experiment where you're convinced that people buy your books solely due to your name and not on the quality of writing.
All of these, and countless more, are reasons for you to write under a pseudonym. The choice is yours. If you do, you're in the company of grand writers such as Mark Twain.
What are some other reasons you could think of to write under a pseydonym?