You might want to mark this as a spoiler thread.

(SPOILER)I personally think that the great cataclysm was hinted at in FFTA2. When you first encounter the Neuhkia, you get told by Lezaford that the opening of a Rift allows through tremendous power and usually great evils. He also says that no Rift has ever been fully opened, and even he does not know what would happen if it did. Ancient loremasters feared the Rifts and built tools to seal them away, including the Grimoire of the Rift carried by Luso.

My guess is the cataclysm was the full opening of a Rift. Specifically, the one connecting to the otherworld of the scions, or the Lucavi, as we know them in the FFT Ivalice. In FFTA2, we are told that the scions (the 13 espers of FFXII, including Ultima, Zalera, Hashmal, Cúchulainn, Adrammelech, and Belias, the Lucavi of FFT) exist in an otherworld, and that their power when summoned by pact in this world is but a shadow of their true strength. When the Rift opens, the Lucavi descend upon Ivalice, unleashing their true strength.

Or maybe not, who knows?