There are two possible causes.
1) An automated process is run each day and if a student’s assessment is 2 months old (60 days) and unfinished, it is terminated. The rationale behind this is that assessments lose their validity after 2 months, as they are intended to capture current abilities at the time of completion. Generally, for a diagnostic assessment to remain valid, it should be completed promptly, with a recommended time frame of no longer than 3 weeks. Assessments that are queued and in the "ready" state will also be taken out of the students' queue after 2 months.
2) If a student is assigned to multiple classes spanning multiple teachers, then it's possible one of the teachers made changes to the queue for the student in question. Additionally, principal role users have the power to log in as teachers assigned to the sites they manage. These users can also make changes.
3) Once an assessment is terminated, a student will need to take the assessment again.