Probably something related to video games. We got kek out of the laughing of Horde players from the view of Alliance on WoW. In a parallel to that the onomatopoeia for laughing in Korea, a huge market for video games, was also kekeke. That's how that took root. I could be wrong though. With the way things are going some tumblrfag might make a different way of laughing because it's less oppressive.