The most popular and widely accepted theory about the origins of the Spellplague was that this calamity was caused by the death of Mystra, the goddess of Magic. For eons, magic had been bound within Mystra's Weave, the universal structure of arcane forces, and her death tore the Weave apart and unleashed the raw magic contained within it, creating the cataclysm that was later known as the Spellplague.
Other scholars, however, didn't consider this to be a logical explanation. Historically, two earlier incarnations of Mystra had died before only to be reborn, and the Weave maintained its hold over the power of magic for the duration of her deaths. In these scholars' views, the truth would never be fully known to mortals, and perhaps not even to the gods.