Maybe a chemical compound that makes the Earth much hotter, and somehow the data centers, the factory, the robots thrive in such a high-temperature situation, and the people who mastermind this operation somehow believe that they can either hide in space or become transhuman in some way, and then basically effect a total baking of the Earth to their benefit and to everybody else’s detriment. So if you want to talk sci-fi, I think that’s easier to explain than nanobots.