Could end up being over oil a la Fallout as the world's oil resources begin to dwindle. I'm not sure what kind of timetable the so-called experts have on that, but I could see it being a cause.
I could also see a major act of aggression by a hostile nation as the cause as well. An example of that being N. Korea making an overt attack on the south in which case the U.S. would be obligated to respond due to agreements we have with S. Korea.
I had have to do so some serious brainstorming to layout the various scenarios but those are some possible examples. Final outcomes and what not are a bit harder to determine...maybe I'll have to come back to this a bit later