There are a number of ways that the S can HTF but it seems to me that a common theme would be that the economy collapses in conjunction with pretty much any of them. The economy could tank on a standalone basis too but it could come about from a major ebola or flu pandemic, from anything that brings down the grid for more than a day or so (cyberwar, terrorism, or some conventional mechanism of what would be WWIII). Looking at the drought out West reminds me that it is also possible that mother nature could cause stresses enough (public water supplies going dry, crop failures etc) to tip the economy.
The world has been in a slow decline for years which allows for adjustments that keep the music playing so to speak. TPTB have been real good at that. It’ll take a sudden jolt to bring it down I think. When it happens it will happen fast. This is why we need to be concerned as to what magnitude event can ISIS or one of the others bring about.