The rise of conservative media, probably. You’ve had decades of fox news and talk radio doing their thing, and now we have stochastic terrorism.
Couple that with the concentrated conservative efforts to reduce the quality of education in America - less educated people are more susceptible to those conservative media tactics. They can’t keep control over all of the craziness they peddle, look at the House of Representatives - they lost the plot and now we are stuck dealing with the shit show they cultivated.