Project complexity has been researched much. The majority of research on project complexity is descriptive and deploys a retrospective perspective on projects, where project are seen as final objects. The use of an ex-ante approach, use for assessment in the initial phases of projects, is less commonly seen in research but has got some attention. However, for the involved managers, the project complexity is a lived experience as the projects evolves in dynamic interactions with the stakeholders. Not much research has focused on this perspective. This paper presents a theoretical framework explaining the unpredictability of events course by project complexity. The theorizing is based on critical realism and focuses on the identification of generative mechanisms as the methodology. The paper then discusses the potential value of the proposed theory - both to the research of project complexity and for practitioners of project management.