TUsing the 2022 Twitter to Mastodon migration as a case study, this article contributes a new understanding of social media migration (SMM). It begins with a review of existing studies of SMM, showing how migration is often understood as a combination of ‘push and pull factors’. We suggest a need to widen the conceptual scope for how we approach SMM in ways that more directly tie such movements to specific questions of power, agency and events that ripple through digital cultures. Drawing on social media account analysis, a survey of recently migrated Mastodon users, content from high-profile Twitter users and other media commentaries, we re-present the migration in order to detail our ‘eventful’ theory of migration. Eventful social media migration is comprised of five elements: an initial X factor; the emergence of a critical voice; a collective platform consciousness; an observable migration; and a wider terrain transformation.