The financial crisis of 2007-2009 was marked by widespread fraud in the mortgage securitization industry. Most of the largest mortgage originators and mortgage-backed securities issuers and underwriters have been implicated in regulatory settlements, and many have paid multibillion-dollar penalties. This paper seeks to explain why this behavior became so pervasive. We evaluate predominant theories of white-collar crime, finding that those emphasizing deregulation or technical opacity identify only necessary, not sufficient conditions. Our argument focuses instead on changes in competitive conditions and firms’ positions within and across markets. As the supply of mortgages began to decline around 2003, mortgage originators lowered credit standards and engaged in predatory lending to shore up profits. In turn, vertically integrated mortgage-backed securities issuers and underwriters committed securities fraud to conceal this malfeasance and to enhance the value of other financial products. Our results challenge standard economic models, and we consider implications for regulatory standards based upon them. We also discuss the overlooked importance of opportunistic behavior to the sociology of markets.