I think most people are feeling the conflict between what is still socially expected of them (start a family, buy a house, have a successful career) and the reality that those days are increasingly out of reach for the average American. They (particularly white men) feel like they've done what they were supposed to do and aren't receiving the upward-mobility rewards enjoyed by their grandparents.

They feel gypped of a lifestyle they felt was promised to them, and are lashing out at anyone they have a sense of authority over. Retail and fast food workers, teachers (also spouses and kids) become easy targets because it's hard for them to walk away from the assailant. And there are almost never repercussions for these attacks, so they continue and can easy escalate and spread.