Adagio's pride causes a fight between her and her sisters. Without them by her side, she gets into a physical altercation and has no choice but to turn to her mortal enemy for help. Luckily, Sunset already knew Adagio was worthy of reformation; her only challenge would be convincing Adagio of that, too.
This is mostly true to canon, placed a few months after Rainbow Rocks storyline-wise, but a few edits have been made. Sunset was born in the human world and ended up as an orphan and then homeless for a few years before she started attending Canterlot High. If I decide to continue this story, that'll be expanded upon but it probably won't happen.
Basically just some dumb Sundagio brainrot.