Well, mine has a few reasons.
First, long ago, there was a series of games titled Megaman, who had a whole bunch of characters, like Fireman, Elecman, Iceman, etc. Shadowman was always my favorite
And secondly, in real life, I am quiet type of personality, and never really speak much at all, but despite that, observe, analyze and clearly understand things going around me. Quiet, but having deep inner world, at least that's what people told me
So I mainly go unnoticed, therefore staying in shadows, so to speak. Which is why I think the name suits me, therefore I use it in most places I register