Twilight's head exploded as she tried to write several thousand chapters at the same time. Her quill proceeded to explode too from the speed at which she was attempting to write at, and then the paper exploded for the same reasons.
The desk then exploded because it didn't feel like desk-ing anymore while Sweetie Giraffe obliterated the rest of the castle for completely unrelated reasons. Ponyville exploded after that because super-giga-hyper Discord decided it would be funny to make thing go boom. Unfortunately, thing that go boom was Ponyville.
Also at some point the sun exploded too but dangit Equestria is magic and magic don't care about your dang heccin' supernovas.