Because I had absolutely no idea what to choose as a username, and just decided to choose a random sound.
My first username (one I used on a couple of online games, like runescape) was "bloople" (although it probably had more o's).
I was really bad with remembering the username/password too, and I didn't play the games too regularly, so it wasn't uncommon I'd come back to Runescape or w/e and have to create a new account, with a similar name (often the same name, with more o's)
That was when I started using "Sir", or "Master" before the name, to come up with other alternatives.
And at some point, I basically got bored of typing bloooooople with additional o's, and just used dooble.