We know that excessive risk taking was a factor in the financial crisis, but why people were willing to take on excessive risk?
There are several explanations for this. In one class of models, misperception of risk generates excessive demand for risky assets, housing and financial assets in particular. There are a variety of stories about why risk is misperceived, ratings agencies failed to do their jobs, risk assessment models turned out to be wrong, people believed that housing prices would continue to go up, and so on. The key is that in this class of models the misperception of risk gives people a false sense of security, and induces them to take on more risk than they can handle.
In another class of models, risk is misrepresented. Here, there is out and out fraud or other practices where, essentially, people know that the house is made of cards, but advertise it as being made of bricks anyway, and assure people that it is perfectly safe. Fraud could cause the victim to misperceive risk, so this is related to the first class of models, but I am trying to separate excessive risk taking brought about by intentional misrepresentation from excessive risk taking brought about by errors in judgment (or, perhaps more accurately in some cases, from negligence).
In a third class of models risk is misallocated, and there are two strands of risk misallocation models. In one, the mechanism that causes people to take excessive risk is knowledge that the government will step in and cover any potential catastrophic losses (risk is reallocated from the private to the public sector). This is the moral hazard problem, and the claim that government intervention led to excessive risk taking has been leveled pretty much wherever government has played a role in housing and financial markets, even when the role has not been very large.
Misallocation can come from moral hazard (government is seen to guarantee the risk), agency issues (originators are not the ultimate barers of risk) or government pressure on institutions to provide more loans to minorities.
Tyler Cowen suggests that the LTCM bail-out may have been a cause of increased moral hazard.