Mine came from the necessity of creating a username that wasn't already in use, nor over 10 characters. (Yes, it was an AOL thing, but heck, I didn't have a computer and was using my brother's Win95 machine to discover the wonders of the internets.) My (younger) brother was a comic freak and had posters all over his room. Not being able to think of anything creative that wasn't already in use, I deferred to one of his Batman posters for inspiration.
The AOL username went the way of the dodo after I bought my own machine, but I decided to resurrect it in later years when I finally stopped lurking over at DSLReports and registered.