I tend to think social media is probably #1 contributor. After all, except for a five year interlude of unpleasantness, we made it about 220 years without otherwise truly being at each other's throats. At a certain level, I suppose 'peace and prosperity' is a contributing factor - consistent with the thesis of the social science above -- inasmuch as we've often had a third external 'tribe' to unite against in our history as an alternastive to being at each other's throats.
A really interesting take on this issue as a political history matter is SP Huntington's great book American Politics: The Promise of Disharmony. While the book is a little dated in its cold war mindset in spots, his basic thesis is that (i) americans are defined by nothing more than a set of shared values (eg, liberty, equality, limited government, opportunity society), (ii) those values are always in tension with each other and there is a continual debate about their 'weighting' in our polity, (iii) given that we are united by nothing more than those values and that they are in tension, and there is a natural historical cycle where, from time to time, Americans are unwilling to tolerate the gap between their ideals and their institutions and (iv) during those periods of "credal passion" major disruption, 'rebasing' and change takes place (eg, the 1830s, the civil war, the progressives, the 60s). While I'm perhaps being a little alarmist, i worry a bit that our current period of credal passion is a little different in that we have shifted a bit from what the underlying mix of values is to whether there's even consensus on some of them. One can speculate over the causes of that (if my fear is true), but if it is true, that's actually a pretty scary thing for a country of our size, scope, and diversity. BTW, Huntington's 'remedy' is simple self-awareness - recognize the values, acknowledge the inherent tension and the cyclical nature of things, and embrace the healthy debate that comes with it in context (which is sort of why i come to this site).