I should say “my” theory. People are funny like things about word usage, just ask Joss Whedon. So yes, faithful reader, this is only my personal theory. Feel free to dispute that in the comments or throw in your own two cents, but know that I am not claiming this theory is shared by others.
Why use a pen name? Why not just own your work and be happy that people can read it? My main theory is that it has to do with the adage, Don’t Write it If You Don’t Want Someone To Read It.
In the wake of the Columbine shootings, any kid with even the slightest trace of eccentricity or originality was victimized by their peers and school administrators. Just ask Brandi Blackbear. She was alienated and harassed by her classmates, teachers, and principal just for exploring the existence of Wicca and her case made national news. I had my own issues in high school but I’ll never forget her story, which was first printed in Seventeen Magazine, and later adapted into a film for Lifetime. (To this date it is the only Lifetime original movie I have ever watched. It’s also the only time I’ll ever pick up and/or read Seventeen.)
What this taught a lot of kids at that time was that if we’re going to write things down, it’s best to keep them as far from prying eyes as humanly possible. If we’re going to write something that might get us into trouble but we still want to have it close at hand, we have to make sure no one can connect us to it.
Writers of erotica tend to use pen names. Anne Rice published her trilogy of erotic novels under the pen name Anne Roquelaure. Although I’ve never asked her about this and she never told me, or presumably anyone, exactly why she used a pen name, I imagine that most other authors with a family, or employers, would use one for this purpose to keep anyone close to them from guessing that such a risque work might be theirs.
Another reason might be that you’re experimenting with something. Suppose Michael Crichton wanted to take a break from dinosaurs, nano technology, and Vikings to write a children’s story about fluffy bunnies that drink tea with leprechauns. Well, he sure didn’t want his hardcore science fiction readers to know that book was written by him. So maybe he published the book under the name Beatrice Montague. This way his good name in literary circles would be intact and, if readers did know it was him and they didn’t like the story, he could always say, “Well, I wasn’t in the right frame of mind to write that story, that’s why I chose the pen name.”
Again, these are all theories. Michael Crichton, if you’re up there, I mean no slight against you and hope you have the sense of humor not to haunt me. (But if you do haunt me, you and I are going to have a long conversation about Pirate Latitudes).