The Everfree forest is just like any other forest—except it's full of monsters and weird magic junk, but ignoring that—it sometimes grows out of control and needs to be trimmed a bit. In order to keep the forest from growing too large, Celestia asks Twilight to start a controlled forest fire to burn a portion of it down. It seems like a simple request, but a series of mishaps and misunderstandings lead to some miscommunication and, subsequently, to Twilight setting fire to all of Equestria.