So every Calamity is paired with the rejoining of one of the shards. But all of the Calamities we know about were motivated by very specific events in the history of Eorzea - the Crystal Tower being overloaded and causing an earthquake, the War of the Magi causing a huge flood*, Dalamud being brought down.

So, is another shard just randomly ****ed whenever something really bad happens on Eorzea? There one moment, gone the next, without any warning beforehand, because everything important is happening on the Source? Makes sense I guess, but then I hear talk about the trouble on the First potentially leading up to a Calamity, which would flip the usual chain of causation...

*I've heard conflicting stories about this; either the Elementals got really triggered and caused the flood themselves, or it was a natural result an elemental imbalance caused by Black and White Mages using every type of elemental magic but water. Which is it?