One of the more common notions about the housing bubble is that it was caused by political pressures to increase homeownership. The Antiplanner’s view is that it would be more accurate to say that the bubble was caused by the conflict between policies aimed at increasing homeownership and policies aimed at reducing homeownership (or, at least, single-family home construction). It would be even more accurate to say that the policies aimed at reducing single-family home construction started the bubble, while some of the policies aimed at increasing homeownership made it worse.
As the Antiplanner noted in recent posts, a lot of factors contributed to the recent housing bubble and subsequent financial crisis. But only two factors were so crucial that, without them, the crisis would not have happened.