Given a multiverse full of infinite possible realities, it stands to reason that at least one version of Twilight Sparkle would end up as a magical anthro-cat schoolgirl ninja. That's just *physics*, y'all.
Twilight Sparkle inadvertently causes an inter-dimensional royal incident. Rarity and Applejack spend the day together. Rainbow Dash makes a new friend. Sweetie Belle drinks tea. The world ends.
A magician never reveals her secrets. The Great and Powerful Trixie just happens to have a few more secrets than most magicians. For example, she’s actually a princess of Transylmaneia. And she may or may not have doomed Equestria to eternal night.
Celestia has plunged the World into eternal daylight. Luna is defeated, the Crystal Empire trapped, and communication impossible. So clearly, the Cutie Map calls Twilight and her friends to solve the problem. Its priorities turn out to be debatable.
Trixie's wagon wheel breaks, stranding her in the middle of Hoofington. Unable to fix it herself, she must call upon Equestrian Roadside Assistance for help. If help will ever come, that is. Which it probably won't.
Golden Harvest always takes pains to be the sensible one for others, but that's easier said than done when Derpy has something "ambitious" planned for Nightmare Night.