Why do parties change candidate lists between elections? Although candidate list volatility is an important indicator of the responsiveness of electoral representation, it has received little attention in research. We offer a critical case study of party list volatility in Finland, using a candidate-centred open-list proportional (PR) electoral system with ideal conditions for ‘ultra-strategic’ party behaviour. Our explorative two-stage research design begins with party elite interviews, to extract factors that can affect list volatility, which in the following step are tested in a regression analysis of 564 party lists in parliamentary elections 1983–2019. Our results show that list formation is a complex phenomenon, where demand and supply factors interact in a contingent fashion. Following trends of voter dealignment, personalization and ‘electoral-professionalization’ of parties, volatility has increased over time. Electoral defeats and declining party membership increase volatility, but a member-driven mass-party heritage that limits party elites’ strategic capacity has a stabilizing effect.