What caused the US housing crisis? The likely answer is: many factors contributed. In the US, the federal government backs the majority of home mortgages. So its effect is large. Another factor is the Federal Reserve's actions on interest rates. Another factor was the rise in oil prices. And other factors like demographics, job trends, and foreign trade were partial causes.

The direct effect of Fannie Mac and Freddie Mac was large in expanding then collapsing the sub-prime market. This graph from the NYTimes shows how their accounting scandals in mid-2000s greatly diminished mortgage backing.

Fannie and Freddie both had multi-billion-dollar accounting scandals. Franklin Raines was CEO of Fannie Mae from 1998 to his ouster in 2004. The Clinton-appointee later paid a fine of nearly $25million. When Fannie and Freddie slowed backing mortgages, the sub-prime sector rapidly deteriorated because buyers could not get loans to purchase homes that previous sub-prime buyers now wanted to sell. The pyramid quickly crumbled.