It's actually a semi-funny story. TL;DR: It was given to me.

The story goes like this:
I love Star Wars, and always thought the Sith were way more interesting than the Jedi. Villains in general are more interesting than the heros who take them down (purely because most authors lack the imagination to write a villain who wins!). As a young teenager I had delusions of grandeur and had begun writing a book, the main character's name was "Lukan", the kind of villain I had always wanted to see in movies, ruthless and a total badass, like a cross between Hannibal Lecter and Darth Sidious. I had shared some short stories from the still in-progress book with a few of my fellow soldiers who were interested in reading something different to pass the time when we weren't out on missions. One day, we were all having an especially bad day and I was taking a few things pretty hard when someone had the bright idea to try to see how far I could be pushed. Lots of shouting and some fisticuffs later, our commander came out and asked what happened. One of my fellow soldiers described the situation leading up to the fight and described what I did as "...he went all 'Darth Lukan' on <insert name of soldier here>..." After that, everybody referred to me as "Darth Lukan", to the point where the commander had my official radio call-sign changed and the name followed me home. My wife even sometimes uses it as a term of endearment.

I keep using it because it was essentially given to me by a soldier whom I respected a lot and counted as a close friend, and every time I see it or hear it, I'm reminded to not let someone else control my "calm". Luckily, I've never encountered a service where someone already had taken the name, so far as I know, I'm the only "Darth Lukan" on the internet (not that I deviate from certain areas very often or anything). So, yeah, that's it.