Here's the closest thing I can find to a root cause (other than an 82% vaccination rate in the county at the center of it).
The first part of that claim is correct. Measles was
eliminated in the United States in 2000 and it was
eliminated across both North and South America in 2016.
Elimination means cases can still occur, but the disease isn’t being continuously spread for a year or more in a specific area.
The second part of the claim, however, is incorrect.
The virus has been brought into the U.S. by people who have traveled to places where there is an outbreak or where the disease is still common,
such as parts of Europe, Africa, Asia, and the Pacific,
according to the Centers for Disease Control and Prevention. From those travelers, the disease can then spread in U.S. communities that have unvaccinated people,
according to the CDC.
For example, the New York City health department
declared a public health emergency on April 9. That measles outbreak, which started in 2018 and spread in the Orthodox Jewish community, was brought on by travelers who had been in Israel, where a large outbreak is occurring,
according to the Pan American Health Organization.
This year marks the largest number of measles cases since the disease was eliminated in the U.S., according to the CDC, which issued a
statement in April identifying misinformation about vaccines as a “significant factor” contributing to the outbreak. Similarly, the executive director of UNICEF and the director general of the World Health Organization issued a joint
statement calling measles “the canary in the coalmine of vaccine preventable illnesses.” They, too, cited online misinformation about vaccine safety as a contributing factor in the rising number of measles cases in high and middle income countries.
