Language. Nativists favored English-speaking Americans over Americans who could not speak English. For the largest part, the immigrants who could speak English came from the northwest of Europe (Ireland, Scotland, Scandinavia, etc.).
Race. The wave of immigrants coming from Europe between the 1880s and the 1920s introduced more racial and cultural diversity, with many Americans considering southern Europeans as non-whites. Nativists rejected this ethnic intermingling.
Job competition. After the First World War, the end of the war effort meant that manufacturing decreased and jobs were more difficult to come by. The wave of Southern and Eastern European immigrant workers at the time added up to the migration of African Americans to the industrial cities of the North of the U.S., and Americans ended up with a lot of competition for few jobs.
The First Red Scare. After the war, many Americans viewed the immigrants from Italy, Spain and Russia as a threat, because they might bring to the U.S. their revolutionary ideas. The rulers of Spain and Italy had been assassinated at the turn of the 20th century, and the Bolsheviks led a revolution in Russia in 1917. The resulting fear of anarchism which spread in the native-born American population at the beginning of the 20th century is referred to as the First Red Scare.
