Stephen Meister argues that government caused the housing bubble:
Federal policy was the chief cause of the crisis. Prior to the end of World War II, the percentage of US home ownership ran well below 50 percent; after the war -- with Veterans Administration assistance to returning GIs -- we saw it climb into the low 60s. But, as the chart above shows, it didn't skyrocket until mid-1990s, when the Clinton administration began pushing its "affordable homeownership" agenda.
In 1996, HUD set an explicit target, commanding that 42 percent of the loans bought by Fannie and Freddie be to people with incomes below the area's median. That target rose to 50 percent before Clinton left office -- and was pushed even higher in the Bush years.
Meanwhile, Washington used the Community Reinvestment Act to muscle banks into making loans to minority borrowers with poor credit ratings who put down miniscule down payments.
On the other hand, one could say that the bubble would not have happened absent government pushing banks to loan more to people with weaker credit is like saying that the machine gun would not exist if gun powder did not exist. That's probably too strong, but it seems more reasonable to say that pushing banks to loan more to less qualified borrowers was a necessary, but not sufficient explanation of the bubble. It's a necessary, but not sufficient condition for the financial crisis.