It was the difference of the new immigrants and their determination to hold on to the customs of their native lands rather than to assimilate into American culture which raised the ire of many Americans.
The immigrants who were part of the new immigration came primarily from southern and eastern Europe. Many were Jews, others were Polish, Russian, Lithuanian, or Greek. Some were orthodox Christians, still others were Roman Catholic at a time when most Americans were primarily protestant. Upon arriving in America, they tended to live in ethno-centric neighborhoods, spoke their native language at home and even on the street with fellow countrymen, and retained many old country customs. This to most native born Americans was simply unacceptable, particularly their religions. As a result, they were forced to live in tenements, (often in filthy conditions) were kept from all but the most menial jobs, and were soon taken advantage of by political machines. Overall it was the belief that the immigrants were "taking over" that aroused the most opposition.