In short, unusually long periods of economic stability lull investors into taking on more risk. This leads them to borrow excessively and to overpay for assets. A Minsky moment is the point in the cycle when investors have cash flow problems due to spiraling debt they have incurred in order to finance speculative investments.
5  Raghuram Rajan in “Has Financial Development Made the World Riskier?”, NBER working paper, 11728, November 2005; presents one of the first accounts of how the growing complexity of the financial system determines its fragility.