On cue, 26-year-old YouTuber Tyler Oliveira published a 40-minute video, “Inside the New York Town Invaded by Welfare-Addicted Jews,” a haphazard exposé of Kiryas Joel. Mr. Oliveira approaches townspeople on the street, asking what they do for a living, how many children they have, and how they support so many kids. Within a couple of weeks, the hostile video and clips from it racked up almost 30 million views across social media, setting off virulently antisemitic reactions.