All speculative bubbles share certain traits: the abandonment of caution, the euphoria of seemingly endless gains, the eventual re-connect with reality (i.e. the bubble pops) and bubble symmetry as the waterfall decline mirrors the euphoric ascent.

At the same time, the specific origin and nature of each bubble is unique to its era and circumstance. The housing bubble that popped with such devastating consequences in 2007-08 was the result of the vast expansion of subprime mortgage lending which began as a progressive goal of expanding home ownership to the lower-middle class by lowering credit standards.

This ideal quickly morphed into the explosive growth of entire industries exploiting this new pool of subprime borrowers via outright fraud: no-document loans (a.k.a. liar loans), negative interest mortgages, interest-only mortgages, etc., all fueled by the packaging of guaranteed-to-default mortgages in mortgage-backed securities fraudulently veneered with a low-risk rating issued by captured ratings agencies.

The rock that starts the landslide doesn't have to be all that large. The entire subprime mortgage sector was a relatively modest percentage of the mortgage market and a tiny slice of the global financial system, but it is the nature of bubbles to be a pyramid house of cards in which the entire bubble is based on debt expanding on the supposedly solid foundation of rapidly rising collateral: the McMansion that sold for $300,000 a few months ago has doubled in value and now supports a $500,000 mortgage.