What changed was that they were free-riders on the rest of the population and one day the rest of the population no longer met the threshold.

One could say it is because of the spread of misinformation and that might be the proximate cause.

But if a drug addict periodically overdoses and needs naloxone, and one day a supply chain issue makes it hard to access and he dies, did the supply chain kill him or his drug addiction? Perhaps monocausal explanations are insufficient.