The Reason for the Mistsickness
So, it finally comes out. I wonder at this numbers plot, as I think many readers will glaze over it and ignore it. I think others will read into it and figure out what it means very quickly, then feel that the reveal here isn't much of a revelation. Hopefully I'll get a majority in the middle who read the clues, don't know what they mean, but are happily surprised when it comes together. That's a difficult line to walk sometimes.
What is going on here is that the mists are awakening the Allomantic potential inside of people. It's very rough on a person for that to come out, and can cause death. Preservation set this all up before he gave his consciousness to imprison Ruin, so it's not a perfect system. It's like a machine left behind by its creator. The catalyst is the return of the power to the Well of Ascension. As soon as that power becomes full, it sets the mists to begin Snapping those who have the potential for Allomancy buried within them.
Many of these people won't be very strong Allomancers. Their abilities were buried too deeply to have come out without the mists' intervention. Others will have a more typical level of power; they might have Snapped earlier, had they gone through enough anguish to bring the power out.
My idea on this is that Allomantic potential is a little like a supersaturated solution. You can suspend a great deal of something like sugar in a liquid when it is hot, then cool it down and the sugar remains suspended. Drop one bit of sugar in there as a catalyst, however, and the rest will fall out as a precipitate.
Allomancy is the same. It's in there, but it takes a reaction—in this case, physical anguish—to trigger it and bring it out. That's because the Allomantic power comes from the extra bit of Preservation inside of humans, that same extra bit that gives us free will. This bit is trapped between the opposing forces of Preservation and Ruin, and to come out and allow it the power to access metals and draw forth energy, it needs to fight its way through the piece of Ruin that is also there inside.
As has been established, Ruin's control over creatures—and, indeed, an Allomancer's control over them—grows weaker when that creature is going through some extreme emotions. (Like the koloss blood frenzy.) This has to do with the relationship between the Cognitive Realm, the Physical Realm, and the Spiritual Realm—of which I don't have time to speak right now.
Suffice it to say that there are people who have Snapped because of intense joy or other emotions. It just doesn't happen as frequently and is more difficult to control.