By Peter Wood, who I feel is qualified to discuss the subject. In addition to being an academic--president of the National Association of Scholars--he published A Bee in the Mouth: Anger in America Now in 2007.
But I would offer a different explanation, in four parts: 1) the decline of formal religion, 2) the aging of the Baby Boomers (while not completely accurate, there is probably something to the stereotype of old people getting cranky), 3) too much big govermment, which imposes conditions and makes choices that people have a lot of difficulty escaping or opting out of, and 4) the increased willingness of institutions to tolerate--even indulge and respond to--anger. In short, it often works.
I realize that all but #2 are endogenous and require explanations themselves. But that might be for later posts.
Postscript: Megan McArdle takes a turn at explaining. She has four causes, too. I like--because it is consistent with #3 above--". . .ever-increasing centralization of the federal government has exacerbated this divide, pushing power to remote authorities that are less accountable to individual voters, and less trusted." And Gurri's idea has promise. But the other two are meh.