The backstory is that Twilight’s found a new spell that miniaturises ponies. Trying it out she discovers that anypony subjected to this ends up feeling compelled to obey another pony. Due to the lack of a target for the authority aspect Twilight ends up latching on to the first pony who find her, namely Applejack. Fortunately Twilight did check that the spell would wear off and she retains her own faculties; it’s just that whenever Applejack gives her an order Twilight can’t help but carry it out.
 
Applejack: You know, Twi, this is much less hassle than goin’ down the spa. When y’are done ya can climb up an’ have a kip on mah chest.
 
Twilight: Uh, thanks Applejack? But what about when your own break’s over?
 
Applejack: Okay. If y’are meant to be mah slave for the day, ya should call me ‘master’. An’ should ah need to get back to work before you’ve had yer rest, Ah’ll let ya ride in mah hat.
 
Twilight: sigh Very well, master. But don’t get too comfortable. It’s just for the day and you’ll be going through this someday, too.