I have argued in the past that rioting and similar forms of mass violence are almost always a matter of license--that is, that a certain fraction of the population are generally eager to riot if the opportunity arises, and will gladly do so if law enforcement authorities start showing signs of being unable or unwilling to catch and punish them for it. An excellent analysis of this phenomenon can be found in Bill Buford's "Among the Thugs", an account of the author's personal experience hanging out among British soccer hooligans during the 1980s. Buford describes the elaborate instinctive dance that a mob of ruffians would collectively engage in with the police, constantly probing and testing to gauge the response, and seizing on any slight failure to keep the crowd completely in line as a chance to break out and begin wreaking mayhem. His key point is that such violence doesn't arise from desperation, or righteous anger, or personal anguish over unhappy circumstances, but rather from many young males' strong visceral attraction to it, combined with the lack of sufficiently forceful efforts on the part of the authorities to stop it.
This exact same dynamic applies to the non-physical battles being fought across partisan political lines these days in the US. Pundits are fond of attributing such events as the mobbing of Republican power couple Mitch McConnell and Elaine Chao, the expulsion of Trump spokesperson Sarah Huckabee Sanders from a restaurant, and harassers disrupting the (separate) restaurant meals of Trump administration officials Stephen Miller and Kirstjen Nielsen, to the desperation of Democrats unjustly deprived of power, or characterizing them as a natural response to Trump's and his followers' own frequent displays of incivility. What these interpretations miss is that incivility is like thuggery: there is no need for an explanation for why people indulge in it, because in fact plenty of people naturally take great pleasure in it. If they are indulging in it today where once they wouldn't have, it's because they believe that they now have license to do so--that is, that their incivility will result in no significant negative consequences for them.
And where does that license come from these days? Primarily from the country's political polarization. In a nation so divided, both sides are "cocooned" with a uniformly like-minded cohort that protects them and even cheers them on while they direct their worst incivilities at the "other side". In other words, like all wars, this one has broken out because both sides, in their insularity, believe they can win.
Of course, at most one side can be correct on that point, and it will likely take some sort of resolution--either a victory by one side, or an unmistakable, prolonged stalemate--to persuade one or the other side to sue for peace. Until then--or until the extreme isolation between the sides somehow begins to ease of its own accord, perhaps for economic or demographic reasons--we can expect plenty more and far worse outbreaks of incivility, egged on by like-minded partisans on both sides.