http://finalfantasy.wikia.com/wiki/Seventh_Umbral_Era

So reading up on info provided from this link.

Information on periods prior to the Fifth Umbral Era is hard to come by, and little is known about the other eras beyond their elemental alignments and the destruction of the Allagan civilization on the Fourth Umbral Era.
The Fourth Umbral Era of Earth - Destroyed the Allagan civilization.
So we can assume the Allgans were destroyed by something dealing with the Earth and not by Bahamut. All that power to seal Bahamut and they lose to an earthquake