This structure is often used recently (I think since mid-2012) in a sarcastic or humorous way, or to indicate that the reasoning is not sound.
a) “Ok, I really want to hang with her because FABULOUS.” (Twitter).
b) “Do you ever meet people who automatically inspire you to be a better person because wow” (Twitter).
I think it is grammaticalized because it's used in a new syntatic context, but is this enough to account for this process?
Edit: I know "fabulous" and "wow" are not nouns. The name of the structure, however, is named like this by linguistics since it follows a noun phrase.