She's been living in Ponyville for a while; it had been hard for her to accept Twilight's offer, but she'd agreed anyway (she'd nowhere else to go, after-all). Time passes and she realises, no matter how kind and friendly the residents of Twilight's castle are, she does not belong in such rowdy a place.

Canterlot Castle had been the obvious other option, and it's quite an adventure, of that Tempest can be sure, she just didn't realise she'd come out of this with something more than new friends.

(Spoiler—or not really: Celestia x Tempest endgame.)

A Human Alternate Universe of the aftermath of School Daze — Twilight Sparkle's school pretty much failed, her wife, Sunset Shimmer, is working abroad, her son is demanding attention, and the only person who knows how to comfort her is her twenty-two year old student who pretty much feels like her daughter.

(Is this how Celestia felt?)