Being Twilight Sparkle is cool, there's no denying that.
Having flight AND magic, being renown throughout all of Equestria for saving the world multiple times, having the rulers of Equestria make up a spell to re-grow the Golden Oak Library after it was destroyed so that you don't have to suffer the loss of Oaky (that's his name)... It's pretty Awesome™.
...for a while.
Because yeah, believe or not, being Twilight Sparkle gets boring. Really boring. Pinkie didn't even last a week when she tried.
When there's no god to save from their evil self, or no centaur to destroy, or no lord of chaos to turn to stone... life is pretty dull. Even books get boring. Yeah! Books! Because, let's be honest, even in Equestria, books are just fucking books. And when you know every book in your library from memory... well there's just nothing left to do.
That's why, on the 4th Saturday of the week (another perk of being a princess is that you can manage to trick the rulers into changing all of the days of the week for Saturdays), while sitting on her husbench (that one bench she loved so much that she decided to marry it, yet another cool thing about being a princess), she decided she'd had enough. While faking to read one of her countless books to avoid having to talk with that annoying pink thing bouncing down the street (why does she even do that, she can fucking teleport), she suddenly came up with an idea to break her boring routine and turn her life around completely. She would become Applejack.
After all, what better change of life than to go from a Princess to a farm pony? This was the perfect solution to her problems!
...And the beginning of everyone else's.