This article offers a process-mechanism explanation of securitization. To make the case for a process-mechanism account more concrete, I use interpretivist process tracing to explain the crisis episode of the Sun Sea, a Thai cargo ship carrying Sri Lankan asylum-seekers, and the securitization of irregular migration in Canada. Drawing on interviews and grey literature, the article shows how securitization was possible and under what conditions, and argues that ideational dispositions of security organizations induced state officials toward a security interpretation of the the Sun Sea. The article aims to demonstrate that process-mechanism explanations represent a compelling methodological alternative with which to trace and explain securitization. The article sees itself as part of a broader refinement of a sociological variant of securitization theory. It seeks to examine and enhance the contribution that this ‘post-Copenhagen’ approach – its core assumptions and theoretical framework – makes to the analysis of securitization.