When the manager sequence has them write their downward reviews after all other reviewers, there are two conditions that can open their downward reviews.
When an Admin opens manager submissions for the entire review cycle (opens all downward reviews for all manager reviewers)
When all other reviews (self, upward, peer) are submitted about an individual direct report (automatically opens the manager review for that individual)
If you send a review reminder notification before opening manager submissions, managers will be reminded to complete downward reviews for any direct reports that meet the second condition above.