I got a Free Skin from Riot...

Ok, now I'm not mad that I got a free skin. I got Warring Kingdom Xin Zhao and that was a skin I was planning to buy for a long time, but my problem was that it was gifted to me by Riot. I want to know the cause of how I got it. Maybe a good report, being a change player, I don't know something. The gift kind of gotten me curious to know why. Thank You for the Free Skin thou!!!