There are six different aether elements and the first six calamities were primarily caused by one of the six elements become the dominant aether aspect of all the aether in Eorzea. IE: 4th Umbral Calamity happened when an Earthquake produced/caused there to be too much earth aether; 6th Umbral Calamity happened when floods produced/caused there to be too much water aether. Or maybe the earthquake was caused when a lot of eath eather was created at once... in any case, one element plays a large part in causing each Umbral Calamity to happen.

Back in 1.0, many scholars thought that since there had been one Umbral Calamity for each element, that the cycle of Calamities was over and there wouldn't be anymore. Loisoix and Urianger found a prophecy that said there would be more then six calamities and thought that Dalamud's fall would result in the 7th Umbral Calamity. A large part of the early 1.0 story was spent convincing the Eorzean Alliance that an Umbral Calamity was coming and that the Alliance needed to be ready for it. Sure enough... there was another one.

It should be mentioned that Shards that have gone through a Flood of Darkness (or Light) cannot be rejoined to Hydaelyn. The Void is what happens when there's so much Darkness in a Shard, it swallows up all aether. 3.4 is all about preventing a Flood of of Light from happening to another shard. So there can now only be... 13 Calamities maximum instead of 14...