It's based on the AC. Higher it is, more you're going to lose. Chances are it was cast a higher point than you've previously seen.
You're right, M. It was odd in that I find the AI uses its WorldSpells rather quickly, but the Sheaim waiting until late game - a good strategy for them as the AC was very high.
I would still like to see the list of buildings destroyed in the event log, if possible, but maybe it is just too much.