When an influx of people move to a new city or country, even if it's because of totally legitimate reasons (more jobs, cheaper cost of living, better schools, for example) locals tend to denounce the loss of the old, "real" city. Locals tend to perceive the situation as a threat when "outsiders" come in raising real estate costs, increasing traffic, pollution, etc. in a place that already has an established and "well-functioning" population.
With immigration, you basically have people that the community sees as outsiders causing problems. Often times this perception is an easy trap we fall into when there are obvious characteristics that difference these immigrants to the current population, whether it be language, accents, skin color, or other easily distinguishable attributes.
When a certain area experiences a peak of immigration, it's usually because said area can afford it and the resources are available. The issue more often than not, is discrimination.
Hope this is helpful to you!
Cultural assimilation is the process by which a person's or group's culture come to resemble those of another group. The term is used to refer to both individuals and groups; the latter case can refer to either foreign immigrants or native residents that come to be culturally dominated by another society
