Calamities explained

[Edax]

As explained by the Exarch:

To begin at the beginning, then... In the ancient past, a single star was divided into fourteen worlds. [...] Though physically separate, they retain a connection to each other, and the Source especially. Now, let us assume that a given element in one of the shards attains abnormal ascendancy. Just as water will flow from the highest point to the lowest, the excess energy will begin trickling into the Source... And such an influx of aether will of course exert a palpable influence. If the element in question were fire, then drought and wild fires might ensue. If it were ice, one might expect the weather to turn bitterly cold. As aether continues to pour in, such phenomena will become more and more extreme, until eventually, a single untimely event triggers a disaster which "cracks" the barrier dividing the two worlds. What was once a trickle now becomes a deluge, sweeping the shard along to be rejoined with the Source. At the same time, the element which held sway in the shard is unleashed in full, its energies amplifying the original disaster to truly catastrophic proportions. An earthquake thus magnified might strike with enough force to shatter continents; a tidal wave might swell to a size capable of drowning entire nations. These devastating events are what we refer to as "Umbral Calamities."

What happens I believe is that a shard is deliberately unbalanced with it's aspect. As explained by the Exarch, it seems to be a slow process as the shard is drained into the Source until a great increase brings about a calamity.

As for what was suppose to happen on the First:
"Their battles brought them knowledge of a hidden mastermind behind the realm's woes, named the Shadowkeeper. The Warriors of Light were accompanied by an elf swordswoman named Cylva who unbeknownst to them, had her own dark agenda. She eventually revealed herself as the Shadowkeeper, who by instructions of the Ascians Mithron and Loghrif, tempered Ardbert and his companions into becoming the Warriors of Light and obtaining their crystals so that they could push the First to the brink of destruction and bring about the Umbral Calamity by killing her."

So perhaps on a fire heavy world, the Ascians might have someone slayed related to Leviathan or someone trying to maintain the water aether on that shard.