I just got a mystery skin from Riot Games themselves

I logged into the game a few minutes ago, and when I went to click on Play, I got the animation for a gift. I thought that maybe I got one of those Honor banners, but it ended up being a free skin. From Riot Games. Now, before you say it, this isn't the skin reward for good behavior that Riot did a couple of months back. I already got that skin. (Surprise Party Fiddlesticks) I don't know what this is, honestly. I log in and Riot themselves apparently decided to give me Mafia Graves. I didn't read about any announcements or anything saying Riot was giving out more skins, so this is a bit strange. Anyone know why this may have happened?

(Also, to the moderators, if this isn't in the right discussion board, I apologize. I didn't know what to list this under. )