Open online education experiences persistently high dropout rates, and the efficacy of dropout interventions has been questioned. Despite considerable research, dropout reasons are not fully understood, and further in-depth investigation has been called for. Prior qualitative retention studies have frequently relied on smaller samples that are unable to generate deeper appreciation of dropout reasons. Over 200 in-depth interviews were therefore conducted with students that had dropped out of open online education. The probability-based qualitative sample facilitated capture of subthemes down to a 5% incidence level or frequency of occurrence. Thematic analysis revealed 41 subthemes within 10 broad dropout reasons. While the broad dropout themes have been identified previously, the subthemes are new and provide richer understanding. This study also captured students' suggestions for what might have prevented their dropout. Thematic analysis identified 19 subthemes within 5 broad intervention themes that respond to the root dropout causes. Many intervention subthemes address personal and learner context dropout factors that have often been considered uncontrollable and unavoidable. This paper therefore redefines dropout in open online education and offers new insights for improving retention. It also provides a strategic framework for evaluating dropouts and prioritising student-informed interventions that respond to the main dropout causes.