After any type of hold is removed from a mailbox, a delay hold is applied. This means that the actual removal of the hold is delayed for 30 days to prevent data from being permanently deleted (purged) from the mailbox. This gives admins an opportunity to search for or recover mailbox items that will be purged after a hold is removed.
So, did you recently remove a hold from these mailboxes? Or were these properties stuck somehow, even though they were more than thirty days old?