If I'm not mistaken this was due to a large influx of settlers from Germany and Austria, who generally were opposed to the militarism in those countries at that time. On top of that the first major war America was in after they became citizens was WWI, which led to widespread anti-German feelings, which they naturally did not much care for.
You can’t discount the anti-militarism and anti-statism of the Germans and the anti-militarism of the Scandinavians but my people followed this emigration route and their pacificm was based on religion. They were there, to a man, for the various Indian Wars, the Revolution, 1812, the Civil War and WWII. They were not there for what they called “political wars”.