I think that what made socialism more frightening to people after WWI was mainly the creation of the Soviet Union.
I would point out, however, that I am not sure that it was the working class after the war that was really most afraid of socialism. I think it was more of the elites who did the Palmer Raids and such who were really worried.
But to the extent that people in general were more afraid, I do think that it had to do with the creation of the USSR. Before the war, socialism was just some idea. After the war, it was an idea that had driven the overthrow of the Russian czars and it was the idea that the Soviets were trying to export to the rest of the world.
So I think people saw it as more of a credible threat to them after WWI.