A very long time ago, before Kaga was Kaga, Kaga was a very big Vocaloid fan, and Kaga's favorite Vocaloid characters were Rin and Len Kagamine. This resulted in the creation of a lot of usernames based around those characters. One of them was just "Kagamine", and this username stuck. It stuck really well! It stuck so well, in fact, that the nickname "Kaga" became a name that I closely identify with to this day. It bled over into real life, and sometimes feels more like me than my actual first name -- and it's been the basis for pretty much all of my usernames ever since. "Kaga" stopped being just a shortened version of a Vocaloid reference. It was just... my name. I don't even associate it with Vocaloid anymore.
Fast-forward a few years. My Iwaku username was "Kaga-kun", and it was getting stale, so I was looking for a different username that still kept the "Kaga" base, and I was having a hard time settling on something, especially since "Kaga" by itself just felt a bit too short and bland for a full username. At this point in time, I occasionally still heard whispers of the name "Kagamine" throughout the site, spoken by people who had known me since way-back-then. But what surprised me was the way that many people pronounced it. The name is technically Japanese in origin, and the correct Japanese way to pronounce it would be "kah-gah-mee-nay", but most people who remembered my old username apparently thought that "mine" was supposed to be pronounced like the English word "mine". This would've annoyed me back when I actually used that username, but, at this point in time -- I had long since stopped caring about the origins of the name (and, to some extent, actively tried to disassociate the "Kaga" name from said origins), so, I really didn't care that everyone pronounced it that way. :P I actually kind of preferred the "incorrect" version.
And, as you probably guessed two paragraphs ago -- I eventually changed my name to "Kagayours" as a play on the retro "Kagamine" name -- or at least, the "incorrect" way that so many people apparently assumed that "Kagamine" was pronounced. XD