Meaning of "Guess what ? Corgi butt"

It seems the sentence "Guess what ? Corgi butt" is quite popular: a web search provides a wealth of stickers, mugs and T-shirts with the same exact wording (and many cute corgis in different positions).

I have some trouble to understand what is the point of this sentence. Is this a reference to something? What does it mean? In what context can it be used?

Unfortunately a search in the Urban Dictionnary didn’t help, it seems there is no entry for it.