Professor Larry White says it was pro-housing regulatory and lending policy. Professor William Black said that Corporate CEO's had bad incentives.
I say that it might have been the anticipation of technological or taste change that did not materialize as quickly as we thought. I don't think the high and rising housing prices, and the fairly stable housing rental rates, are consistent with either Professor White's or Professor Black's theory.
We'll see what Professor DeLong has to say.
By the weekend, you will be able to read all four essays here.