The Mirroring Preferred Configuration alert indicates that a database mirror partner took on a non-preferred role. This event means the roles in the mirroring relationship do not conform to the roles specified as Normal. This alert does not reset over time and remains until you restore the roles or you change the Normal state. You can restore the roles or modify the Normal definition in the Mirroring view of the Databases tab.
SQL Diagnostic Manager triggers this alert based on the diagnostic data collection interval specified in Server Properties. For more information, see Set general server options.