Lite Sprites/WMG

The Sprites accidentally released the shadows

 * At the start of the special, the girls mention that Astra has to keep the stars lit or else shadows will take over Lite-Topia. In the flashback, the girls found their light wands in a crystal cavern, hanging from the ceiling. As the shadows taking over apparently wasn't a problem before the girls got their wands (Lite-Topia is shown as pretty colorful even before the girls got their wands), one can assume that the wands were in the cave because they were sealing the shadows away.