So, here's the story, Celestia and Luna were talking to Filly Deluna until Deluna lost her temper and used magic to take Celestia and Luna's magic away, causing them to turn into their past selves, but weaker. They got scared because Deluna was becoming a monster and Deluna ran off terrorizing Canterlot. Celestia and Luna were terrified, but they called the guards to go and find Deluna while they wait to get their magic back, so they can perform a spell on Deluna turning her normal and more of their magic comes back to them.