Student behavioural disengagement is a problem in many schools. This paper aims to explain why students' behavioural disengagement occurs and reoccurs in Swedish classrooms in terms of two mechanisms. Mechanisms that explain student disengagement are tested quantitatively and illustrated qualitatively with primary data consisting of 74 video-recorded classroom lessons from three compulsory schools in Sweden. The regressions suggest that peer encouragement and the school subject curriculum are central for explaining student behavioural disengagement. Qualitative analysis decomposes how the mechanisms of peer encouragement and school-subject curriculum increase the risk of behavioural disengagement in real time using transcripts and images. In conclusion, this study shows how both mechanisms provide complementary explanations for students' behavioural disengagement.