THEORY: We've already seen this year's ultimate skin
As we all know, ultimate skins evolve - either with levels, or with skill usage. Ezreal's armor gets more advanced as he levels up.
Now, think about a skin released on the PBE quite a long time ago, that received a significant armor upgrade a few patches later. Can't quite remember? Take a look at this picture (by Surrenderat20.net): http://4.bp.blogspot.com/-7N09AQByd94/U7N0EU9fzFI/AAAAAAAAS70/114PYFcSgdY/s1600/aetrg%5D.jpg
Theory: Riot releases every evolution of Cait's ultimate skin separately to both find bugs and not to tease the new skin at the same time. And why there's still no splash for that skin? And for some reason my reddit posts about this got instantly removed. Coincidence?