Herman Minsky had a theory on this. A quick read of his wikipedia page seems to explain it decently well. Essentially that when credit is easy to acquire and firms have plenty of cash flow to service their debt they will make riskier plays in order to chase better returns. As the market is buzzing along nicely these will initially pay off. This will inevitably end and financial institutions tighten credit availability which causes a contraction. Commonly known as the business cycle.

A Doctoral student of his Randall Wray wrote a book called 'Why Minsky Matters'. Goes into much more detail and comes from a banking perspective rather then my basic explanation.