This paper addresses the question of when and why institutional conflicts break out over decision-making competencies in the European Union. It argues there is untapped potential in constructivist approaches to explain the occurrence of such conflicts. Rationalist institutionalist (RI) models based on the idea of the Commission as a 'competence maximizer', while serving as shorthand for understanding institutional dynamics, risk producing simplistic accounts of institutional conflicts. Tensions between broad collective understandings of policy issues and formal institutional rules can help explain why the Commission initiates institutional conflict in specific instances. This general argument is formulated as a social mechanism termed the rupture mechanism, its potential demonstrated by evidence from an in-depth process tracing study of one of the most controversial institutional conflicts between the Council and the Commission in the 2000s.