Hiding worries or concerns to shield your partner from something unpleasant, or even just to keep the peace?

WSJ: Lying to a Spouse: Relationship Advice on How It Can Be Healthy

There are quite possibly an infinite number of reasons for using a pseudonym, and your assertion that anyone who uses one must be hiding something is moderately insulting.

FORBES: Enders Game, Orson Scott Card, and People who Hide Behind Pen Names