The focus of this study is on the pedagogy of flipped learning (FL) in mathematics teaching. There has been extensive research into FL, but less research on teachers' pedagogical rationales when adopting this pedagogy. The present study addresses this research gap by examining interviews with mathematics teachers in Finland. These teachers identified themselves as FL advocates. A thorough analysis of the teacher interview data inspired by a grounded theory approach revealed three main pedagogical rationales for FL in the teachers' accounts, namely, Individualising Learning, Fostering Self-regulated Learning, and Fostering Engagement. Individualising Learning emphasises attempts to differentiate and humanise learning mathematics in heterogeneous student groups. Fostering Self-regulated Learning highlights the teachers' emphasis on students' responsibility in goal-oriented activity that is supported by self-paced learning. Fostering Engagement is related to the teachers' attempts to create a personally motivating learning environment for students. The results of this study contribute to the research into FL in two ways. First, the teachers of FL view self-regulation as an objective of education, and not just as a means of education. Second, the teachers underscore general learning skills over disciplinary learning in mathematics.