Irrational exuberance in the housing market led many people to buy houses they couldn't afford, because everyone thought housing prices could only go up. The Fed should have raised interest rates in 2004. Low interest rates in 2004 and 2005 helped created the housing bubble. Irrational exuberance set in again as many investors took advantage of low rates to buy homes just to resell. Others bought homes they couldn't afford thanks to interest-only loans.
In 2006, the bubble burst as housing prices started to decline. This caught many homeowners off guard, who had taken loans with little money down. As they realized they would lose money by selling the house for less than their mortgage, they foreclosed. An escalating foreclosure rate panicked many banks and hedge funds, who had bought mortgage-backed securities on the secondary market and now realized they were facing huge losses.