While secessionist movements have surged globally over the past century, the prevailing structuralist and institutionalist approaches inadequately capture the dynamics of secessionism. We argue that lost autonomy triggers, sudden transformative events symbolizing a loss of autonomy for ethnic minorities, can be profoundly disruptive, eliciting pro-secessionist backlashes. Despite their significance, the causal impact of such triggering events remains underexplored. We investigate two typical cases: the 1992 Great Bank Affair in the Faroe Islands and the 2010 Spanish Constitutional Court decision to reform Catalonia’s autonomy statute. Using synthetic control models, we demonstrate that these triggering events were essential for the subsequent secessionist waves, which would not have occurred otherwise. Qualitative process tracing analyses further support our findings, indicating that these events were perceived as highly disruptive, causing significant shifts in public opinion and prompting political responses. Our findings underscore the importance of recognizing lost autonomy triggers as proximate causes of secessionism.