They sense change, and feel threatened.

Americans have no sense of history, for them, its something they read about in books, not something that happens over a human lifetime. When was the last time there was hunger here on a mass scale? When was the last time enemy soldiers occupied our cities? Even our wars are fought far away, by a few volunteers.

Just one example: there are very few people alive today who remember the Great Depression. In fact, there aren't that many people alive today who have actually spent any time talking to people who experienced those days. Even if we care, we have to depend on what we read in books, and they don't agree.

So when people sense big change coming, especially an ugly one, they feel someone must be responsible for it, someone must be to blame, and they must be stopped.
Demagogues are very good at exploiting this fear, and they do it by making people angry.