An oddly regular hazard of attending Canterlot High ever since magic was introduced to the world is that, occasionally, things will sort of… ‘shift’. Objects will be one thing, or in a certain state, and then will suddenly be turned into something very different, changing their appearance, state of matter, or size with an alarming lack of regard for the laws of physics.
 
In this case the victim of this effect was poor Octavia’s skirt. The few remaining scraps of it left have been turned over to the science labs (along with the rest of her outfit, as per close-contact procedures), and the girl herself is awaiting processing alongside them.