This is a "Wild Mass Guess" entry, where we pull out all the sanity stops on theorizing. The regular entry on this topic is elsewhere. Please see this programme note.
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.