Adam J. Levitin and Susan M. Wachter write,

the bubble was a supply-side phenomenon, attributable to an excess of mispriced mortgage finance: mortgage finance spreads declined and volume increased, even as risk increased, a confluence attributable only to an oversupply of mortgage finance.

The mortgage finance supply glut occurred because markets failed to price risk correctly due to the complexity and heterogeneity of the private-label mortgage-backed securities (MBS) that began to dominate the market in 2004. The rise of private-label MBS exacerbated informational asymmetries between the financial institutions that intermediate mortgage finance and MBS investors. The result was overinvestment in MBS that boosted the financial intermediaries’ profits and enabled borrowers to bid up housing prices.

Thanks to Richard Green for the pointer.

My ability to discuss the paper objectively is severely hampered by confirmation bias.