When there is rapid asset price appreciation, many people (including bankers, politicians, and many others) are tempted to leverage and buy into the boom. This helps to prolong the boom. But nothing is forever and asset prices reverse ("bubble" pops) then people are eager to de-leverage and that prolongs the bust.
That is the textbook "credit cycle" which often becomes a bigger "business cycle". That's the dry textbook version. Michael Lewis has a much better name. He prefers to call it "Boomerang".
Lewis is obviously a great writer, keen observer and very smart guy. He makes it his business to go to Greece, Iceland, Ireland, Germany and California to see and hear the real players. He calls it "a new opportunity for travel: financial-disaster tourism." In each case, he manages to tease out delightful insights (and occasional confessions). In some cases, there are also terrific character profiles. His Santa Monica bike ride with Arnold Schwarzenegger is worth the price of admission alone.