It's many different things, but scale and time horizon is certainly one of them. The scale of immigration over just the last 30 years is truly unprecedented, and will have political impacts for the next 50. The share of the population that is foreign born is up to 15% (likely much higher, since it's difficult to actually count illegal immigrants), which is the first time since the mid 19th century it's reached that level. Qualitatively, seeing e.g. a childhood neighborhood turn into something that resembles a (very) foreign country is ... jarring. I'd also submit that your brief summary of American history is wrong, and is part of the problem. It very slyly changes the foundation of the nation from something heroic to something that we should be ashamed of, and that mass immigration is the only way to do penance for that sin. It's fine to advocate for people coming here to seek a better life, but it's wrong to describe America as a "nation of immigrants", when the reality is closer to a "nation of settlers" who built up a largely empty country. I also tend to roll my eyes when people invoke "Islamophobia", since the United States is a) not a Muslim country and therefor does not have to defer to Islamic interests, and b) that term is typically invoked in an attempt to bully someone into agreeing with a more extreme position since, well, you don't want to be Islamophobic now do you.