Growing Anti-vax sentiment results in a gradually falling fraction of immunised population, Measles is an infectious disease with about 20:1 ratio, but obviously incidence is somewhat random, so a population with only 80% vaccination might go five years with no problem, while one with 90% gets unlucky and sees a rash of cases.

This grew as part of a general "Facts aren't true" era in which we've deployed powerful technology that's able to spread nonsense very quickly to gullible people. Trump is just another symptom and his rise to power in the US in particular is also a consequence of systemic flaws in the US government, a known bad model for governments.

The "facts aren't true" stuff is why the Nazis who have decided to protest near me have such a weird collection of beliefs which to them seem more or less equivalent. They believe 5G mobile telephones caused COVID, that trans people don't exist, that migrants are simultaneously both incredibly powerful, almost superhuman, and yet also very weak and entirely useless. None of this has to make any sense at all, indeed if you're the sort of person who wants to make sense of things you're not what they're looking for.