I refer to Jared Diamond's book of that name, in which he argued not only that collapses can happen, but pointed out some mechanisms involved. His beginning example is not Rome or the Incas, but Rwanda, where severe overpopulation and landlocked resource limits brought the tension between two populations to the breaking point.
I feel the angry-white-guy who is now a fearful white guy is experiencing the feeling that caused the genocide in Rwanda. With the white privilege declining, and the people that used to be lower than white guys (brown people, women, foreigners, gays) getting attention and assistance, I think some of these fearful white guys are correct in their fears. Their "country" is going away, they are being outnumbered and outvoted. With flight not possible, fight is the alternative.
I will predict the San Bernardino shooters are twenty-something white guys, perhaps Aryan Nation but probably not, just wishing they were part of a bigger operation to protect "their" country from the impurity of "degenerates", like other races, foreigners, but in this case, the weak and disabled. Let us remember the drive for purity from the Nazis. Let us remember Donald Trump describing a black protester as "disgusting". Let us remember him mocking the disabled NYTimes/ WaPo reporter. The clients at the social-service center were likely both brown and developmentally disabled, precisely the people Nazis wanted to eliminate.
I believe we can include the ISIS problem as a Jared Diamond "Collapse" scenario. Those folks see their future contracting. The feeling of being able to have a country that is not a pale reflection of Western civilization is fading. And to top it off, how stupid do the jihadis have to be to know their only source of wealth, oil, is becoming something reviled and dangerous. Saudi Arabia is going to fail---they can't stay medieval for much longer, and are allowing women to hold office. That won't go backwards, but also in their future is general taxation to run the government. Expect some nasty tensions over the declining revenues from oil and the end of the free ride. How would you feel if you had as your home a hot and sterile desert that had only oil, which many people want you to leave in the ground?
It's getting messy, and messier is the future.
Update: Speaking of messy, a woman involved in the SB shooting. Not sure what that means.
Further update, taking into consideration what On The Media points out--early info is sure to change: Syed Farook identified as a suspect. Could be way different from what I thought, no surprise there.
More: I am growing suspicious that we have seen the expected event where native terrorism has learned the lesson of the surprise small-arms attack. There is no real defense against this, bar eliminating the weapons, or more to the point, the ammo.
Further speculation: Because we, or at least I, don't know much yet, I am looking at the weird news that the apparent perpetrators were young marrieds with a child, and the man seems to have been a colleague of the victims, that they must have laid down some plans because they had lots of gear, but their actions must have been spurred by circumstance. Was this a more generic terror plot that was turned into a revenge act? Did they assume they would get away? (They did--it seems police heard about Farook and staked out the house.) The right tactics would make for a successful getaway.
I note one thing regarding aftermath: White shooters seem more likely to surrender or be taken down without being killed. The Columbine shooters killed themselves and so did Adam Lanza, but Robert Dear surrendered, as did Roof, and Holmes---not to mention assassins that attacked Kennedy(s), Ford, and Reagan. Do white people trust each other more? Do police have confidence in talking to white shooters, and do those people have confidence police are negotiating in good faith?