PMP Exam Simulator

You are managing a project and so far you have been very satisfied with how the project is going. People know when to start work thanks to the work authorization. The issue log helped to keep track of stakeholders' concerns. The sponsor is very happy with the work of the team, so, he has hosted a milestone party. Suddenly you get informed by one team member that the results from the completion of his activity's predecessor are 3 days late. How would you best describe the reasons why it happened?