Although agile software development (ASD) is widespread, the contributions of individual agile practices to development success are still largely unclear. In this paper, we explore the hidden cause effect relationships between the application of social agile practices, the realization of social agile principles, and the resulting contribution(s) to ASD success. To capture ASD success, we consider both the effects on developer acceptance and economic business values. Based on an initial ASD success model and data from a survey of 197 developers, we found that social agile practices such as reflection, business IT alignment, and self-organization seem to particularly promote ASD success. We also found indications that the realization of these principles is primarily driven by practices such as retrospective meetings and shared leadership, whereas prominent practices like daily meetings and pair programming seem to have no effect. Our results thus call for reassessment of agile practices and their use in practice.