In Normal Accidents, Charles Perrow argues that systemic accidents are 'normal' in the sense of being spontaneous, unavoidable, emergent properties of complex systems. Worse, their contributing causes are often obvious in retrospect, but very difficult to foresee in prospect.
Perrow identifies three conditions that make a system likely to be susceptible to Normal Accidents:
The system is complex
The system is tightly coupled
The system has catastrophic potential
While at many junctures the global-industrial-economy-gone-'half-urban'-world is loosely coupled, many of its critical, interdependent systems (finance, power and fuel, transport, communications, industrial processes (including resource extraction) are founded on increasingly tightly coupled components and mechanisms.
A normal accident that takes out a critical sub-system can initiate cascading failures through a process termed 'contagion', potentially bringing down the system as a whole. Re-boot with its own resources is considered by experts in each field to be improbable (for example, a nuclear plant which loses grid power relies on back-up generators, and thus fuel, personnel, etc., until the grid (external) is restored)
Exponential growth within the closed world system and knock on effects are stressing the system as a whole. Local stresses, which might have been weathered at earlier points, can push any system component beyond its tolerance threshold.
Cascading events could play out over any time scale. They are likely cascading as we speak (resource depletion, global warming, salination and loss of soils, etc. can be viewed as normal accidents in progress). Collapse could continue relatively slowly, or it could reach a tipping point (outbreak of nuclear war, say, or algorithmic trading crashing world currencies), and from one day to the next present us with full scale TEOTWAWKI.
To my mind, at least in the short term, collapse due to normal accident is the most likely cause of cataclysm.
PS. David Korowicz also writes extensively about systemic collapse... articles and essays at http://www.davidkorowicz.com