I see the reasons for their anger as two-fold. The progressive philosophy is founded on envy and anger, and attracts people who have those faults.
But first, the author makes the mistakes that “progressives” used to care about people. They never did. Progressive=liberal=socialist=communists always start off by claiming they “care” about the “poor and oppressed” but that’s just their jive talk shtick. It’s ad copy, meaningless.
The US progressives have had a mask on, or some of them have, for a while, and now the masks are off. Angela Davis, Bill Ayers, Abbie Hoffman and plenty more - like the stone throwers the author mentions in Oakland - they had their masks off. Progressives have always been filled with and motivated by hatred, anger, envy, lust for power and the love of destruction. Their philosophy is evil, hateful, destructive and appeals to people who already have envy, greed and desire for power.
And the philosophy of progressivism throws gasoline on those bad traits so they feel entirely justified in indulging in the lowest of human emotions and motivations.
One thing common to progressive/socialist etc philosophies is they are all based on atheism and a rebellion against Natural Law of every kind. That is why they always embrace the foulest of behaviors and practices such as abortion, feminism, perversions of all kinds and hate those who are devout, of any relgious persuasion.
With the mask on, they have duped silly irrational people by acting as though they actually cared about the poor, etc. But now with their masks all off, many of the dupes are wawking up and going to Trump rallies.
Jealousy is a hate based emotion and quite murderous.
Jealous people don’t hate the individual for having what they, themselves dont. They want that individual destroyed because of it.