Scientific research on the banking crisis 2007-08 has answered many important questions according to generally accepted methodological standards. However, there remains at least one outstanding question that has not been answered with methodological accuracy: What caused the severe USA banking crisis 2007-08? To address this question the paper uses a counterfactual definition of 'cause,' distinguishes between separable and non-separable causes, and employs a well-posed methodology for the causation analysis of singular events. In addition, first causes and preponderant causes are distinguished. The main result of this paper is that the preponderant causes of the banking crisis 2007-08 were securitization and ignorance.