The princesses gave their magic to Twilight to prevent Tirek from stealing it. As a result, they end up stuck in Tartarus. The fact that things manage to go downhill here says a lot about the wonders of family.
Discord wants to enjoy the Summer Sun celebration with the rest of Ponyville. Sure, everybody's already dead by the time he arrives, but it's not like that's going to stop him.
Twilight just accidentally killed everyone in Ponyville. Celestia's coming to visit later that day. She has to think of something to keep her from finding out, and fast.