There are many different explanations as to why Equestria fell. The true answer is quite simple: Nothing can kill Friendship more than a mother having to bury her own child.
The only reason Wallflower made it this far is because Sunset was always there when she needed her. How can she possibly win a fight against herself when Sunset's away?
After the Storm King's demise in the Battle of Canterlot, the Storm Kingdom was left in shambles. All across the Kingdom, rebellions broke out, chaos reigning. And as a new King begins peace talks with Equestria, a brutal figure decides to intervene.
Melody Breeze signs up to the Penpal program of Equestrian Connection Project. A mare in despair hopes for a reply. She deadly needs one. From somepony, who cares... It happens that Dawn gets Melody's letter. She dares to write a reply...
Magic being drained from Equestria once again. Equestria on the brink of another civil war. And with her friends dead, is it any wonder Twilight is having a massive panic attack/mental collapse?