I think it started with Hungarian giga chud Victor Orbán using him as the bogeyman in his electoral campaigns. Soros was born in Hungary and is funding liberal projects in the country, including an international university in Budapest that has committed the unspeakable crime of offering degrees in gender studies. Soros is also active in refugee aid. So Orbán is using him to create narratives that are at the same time anti-semitic, anti-refugee, anti-intellectual, anti-LGBT and anti-feminist. It's Elders of Zion meets Great Replacement with a big dollop of postmodern-neomarxist academia bashing on top. Really ticks off all of the boxes for triggering chuds.

For context: Hungary is a possible transit point when refugees migrate from the camps in Greece via the Balkans to destinations in Central and Western Europe. Orbán brags a lot about stopping these people (or, in his diction, the onslaught of a giant horde of barbarians) with his fence and his thugs. He broadcasts videos where women and children are kicked while trying to cross the border and stuff like that. Chuds across Europe lap that up eagerly. For example, you'll see Identitarians in Austria and Germany refer to refugee boats and refugee rescue ships as "the Soros fleet". They seriously think Soros wants to ship in muslims for a mayocide and that Orbán is the last bastion of defense against that plot.

And just as many American fascist narratives are making their way across the pond and give our nazis ideas about arming themselves and preparing for a race war, Trump is referencing European fascist narratives to compare and conflate Muslim refugees with Latin American refugees, portray Hillary Clinton as America's Angela Merkel etc. I think this is how Soros became an established bogeyman among the US right.