Established contributors are the backbone of many free/libre open source software (FLOSS) projects. Previous research has shown that it is critically important for projects to retain contributors and it has also revealed the motivations behind why contributors choose to participate in FLOSS in the first place. However, there has been limited research done on the reasons why established contributors disengage, and factors (on an individual and project level) that predict their disengagement. In this paper, we conduct a mixed-methods empirical study, combining surveys and survival modeling, to identify the reasons and predictive factors behind established contributor disengagement. We find that different groups of established contributors tend to disengage for different reasons; however, overall contributors most commonly cite some kind of transition (e.g., switching jobs or leaving academia). We also find that factors such as the popularity of the projects a contributor works on, whether they have experienced a transition, when they work, and how much they work are all factors that can be used to predict their disengagement from open source.