From an essay by George Soros:
The typical sequence of boom and bust has an asymmetric shape. The boom develops slowly and accelerates gradually. The bust, when it occurs, tends to be short and sharp. The asymmetry is due to the role that credit plays. As prices rise, the same collateral can support a greater amount of credit. Rising prices also tend to generate optimism and encourage a greater use of leverage-borrowing for investment purposes. At the peak of the boom both the value of the collateral and the degree of leverage reach a peak. When the price trend is reversed participants are vulnerable to margin calls and, as we've seen in 2008, the forced liquidation of collateral leads to a catastrophic acceleration on the downside.
Bubbles thus have two components: a trend that prevails in reality and a misconception relating to that trend. The simplest and most common example is to be found in real estate. The trend consists of an increased willingness to lend and a rise in prices. The misconception is that the value of the real estate is independent of the willingness to lend. That misconception encourages bankers to become more lax in their lending practices as prices rise and defaults on mortgage payments diminish. That is how real estate bubbles, including the recent housing bubble, are born. It is remarkable how the misconception continues to recur in various guises in spite of a long history of real estate bubbles bursting.
Bubbles are not the only manifestations of reflexivity in financial markets, but they are the most spectacular. Bubbles always involve the expansion and contraction of credit and they tend to have catastrophic consequences. Since financial markets are prone to produce bubbles and bubbles cause trouble, financial markets have become regulated by the financial authorities. In the United States they include the Federal Reserve, the Treasury, the Securities and Exchange Commission, and many other agencies.
In his book The New Paradigm for Financial Markets, Soros argues that the most recent crisis differs from the various financial crises that preceded it.