Recent trends toward mechanistic approaches offer a new perspective in understanding policy change and stability. This paper analyzes causal mechanisms leading to unexpected policy change by using punctuated equilibrium theory. As empirical illustration, the paper presents a case study on the introduction of the German mandatory lobbying register in 2021 after a 16‐year‐long debate. Methodologically, the paper employs process tracing and qualitative content analysis to examine policy documents. We identify a combination of three mechanisms: end of a de‐thematization of the policy issue, growing dominance of the issue network favoring stricter transparency regulations, and issue validation through the accumulation of scandals. Thus, policy change results from the descend of policy actors defending the status quo while those advocating for change ascend to an influential position, and actively exploit focusing events as fertile ground for reform. The paper contributes to a refined theoretical understanding of the causal mechanisms of policy change.