Quote Originally Posted by foussi View Post
I theorize that there was something inside the planet that caused the chain reaction.

What do you think?
According to the wiki : "At some point, a mysterious "sound" emanating from the depths of their star caused the Ancients to lose control of their Creation magic, unleashing monsters formed from their subconscious fears. "

Now what caused the sound, we don't know. It may have been the result of some malicious design to destroy The Ancients. It appears even the Ascians never figured out the cause in the years after the Sundering and if Emet managed to Rejoin the Star, history may well have repeated itself. I assume the Ascians were complacent because they assume they'll have Zodiark to prevent the effects and thus never looked too deeply into the cause. This is pure speculation on my part.