The current study was conducted with the goal of helping to address the research gap of improving preservice teachers' pedagogical knowledge about probability by investigating how preservice teachers explored concepts of probability during aleatoric music composition, as well as their contemplation process during follow-up reflections focused on using music-centred activities for developing elementary students' probabilistic reasoning. Preservice teachers (n = 71) were guided to compose a piece of aleatoric music, with coin tossing utilized as the technique to generate the tonal patterns. A sum of 421 pieces of qualitative written data was collected. A content analysis was conducted to find the recurring themes and subthemes across the entire set of writing pieces. The overall findings indicated that most preservice teachers' probabilistic reasoning was influenced (for better or worse) by the observed data obtained when composing their music, rather than a conceptual understanding of the Law of Large Numbers and the relationship between theoretical and experimental data. The conceptions -- and misconceptions -- held by the preservice teachers after aleatoric music composition were found to be related to their expressed teaching strategies.