Everyone affected by the mists’ snapping had allomantic potential.
In the annotations for The Hero of Ages Chapter 49 Part 2, Brandon Sanderson says:
Noblemen, despite what Spook says in this chapter, are not immune to the mistsickness. The rumor Spook is referencing does have merit, however. You see, since the mists are Snapping people and awakening the Allomantic potential within them, it will affect far fewer noblemen than skaa. Why? Because a lot of the noblemen have already Snapped. They were beaten as children to bring out the powers.
However, that won’t stop all of them from being affected by the mistsickness, because the mistsickness is also awakening Allomantic potential that would otherwise be too subtle to be brought out. Pretend there’s a sliding scale of Allomantic potential. 100% means you’re an Allomancer—in this series, only two people have hit 100%—Vin and Elend. Buried within a lot of people, however, is enough of a touch of Preservation’s power to hit, say, 50% on the relative scale of Allomantic power. These people, when beaten and made to pass through something traumatic, awaken to their Allomantic abilities.
There are a lot of people out there, however, with something more like 20% to 30%. These are the people the mists are Snapping—since the mists are, themselves, partially the power of Preservation, they can touch people and increase their Allomantic potential slightly and then bring it to the forefront.
Emphasis mine. source (you will probably have to click the "show spoilers" button to see the relevant quotes)
Basically, it was able to snap people who otherwise would not have been able to snap through regular circumstance, awakening the weak allomantic powers in their bloodlines.
You can note that unlike the other answer, this suggests that nobles are not immune to the snapping; but are just statistically less likely to benefit. They have attempted snappings at youth, and the bloodlines are generally strong enough that such a snapping unlocks their power, so it is much less likely for them to have a hidden potential that the mists can tap.