Fact: Twilight Sparkle cannot dance.

Fact: Twilight Sparkle believes if it cannot be fixed with magic, it cannot be fixed.

Result: Twilight Sparkle's terrible dancing has become contagious. Everypony who catches it can't stop dancing like Twilight. It's a race against time to find the cure, or everypony may be stuck like this…

Forever!

[Edited by the superfluous Blue_Paladin42, the erroneous TheMaskedFerret, the humorless SwanSong and the inoffensive WolfVenom]