Somehow, some way, Rarity takes control of a story. With the infinite number of possibilities set before her because of this, she has only one thing in mind for how to change things. She wants to be the element of magic, not Twilight Sparkle. So, Rarity bends the fic to her command as we see what it would be like if Rarity took Twilight's place and threw everything off.
Thanks to Pageturner1988 for making the vector I used as a base for the page image.