In this guide I’ll explain why stability in your cities will sometimes suddenly drop from 100% to 0% in 1 turn and how to fix this. It is usually related to building infrastructure that adds pollution to each of a kind of quarter in your cities.
The most common one is adding 1 or more pollution per turn on each makers quarter in a city. Industrial era infrastructure can do this for both the makers quarter as well as the farmers quarter. Examples of infrastructure that do this are the Sawmill, the Coal Plant, as well as the Factory. The Industrial Silos infrastructure adds pollution to each farmers quarter.
You can fix this by replacing some of the kind of quarter that is now generating more pollution with commons quarters. This both reduces the level of pollution as well as increases the stability based on adjacent quarters. It is usually better to avoid infrastructure that adds pollution per quarter until you have a significant stability surplus in a city. Being forced to replace many of your quarters with commons quarters can overall reduce your yields in a city to below what it would have been without building the polluting infrastructure. There is also late game technology and infrastructure that reduces pollution, such as Fusion Reactors as well as hydroelectric dams and wind and solar farms.
So, in summary, the reason why sometimes stability drops from 100% to 0% in one turn in your cities is likely because you built some infrastructure that adds pollution to each of a kind of a quarter in a city. Avoid building this infrastructure or mitigate the impact by replacing that kind of quarter with commons quarters. More in this guide: Staying at 100% Stability with Hundreds of Quarters