In a way... more a side effect of the astral war. Which boils down to Ifrit getting jealous of the affection Shiva showed us mortals. He decided to destroy us... and Bahamut didn't like him acting out of line. The clash of magic ripped our world apart in ways that are still apparent millennia later.

[He sighs heavily, sliding an arm around Will as though the contact might help him make sense of Bahamut's callousness.]

The Scourge is just one more of those affects.