Just watch the video posted above. Really. Watch the whole thing. You done yet?
That clip, along with similar ridiculous videos, helped popularize the theory, bringing it out of web forums, chat rooms, and discussion threads into articles like the one you're reading right now. There's a whole sub-Reddit devoted to the Cleganebowl, a place to share theories, images, and jokes about the eventual confrontation. For example, Sunday night's episode inspired multiple posts about a single line Jaime said. The thread has its own terminology and in-jokes. They mostly involve two words: get hype.
As you can tell from the menacing "Hard in the Paint" beat that scores the video above, the excitement fans have about the Cleganebowl is fervent. It's like a higher calling -- a religion founded on hype. The type of cause that, were it happening in Westeros, might lead to forehead carvings.