The root cause is a user set the default value of the isManaged (0x1295d) attribute (Maintenance Mode) to "Yes" for the Model Type associated with the model using the Attribute Editor. The model or models selected when the user launched the Attribute Editor will have an event logged stating the model was placed into Maintenance Mode by the user. However, any other model of the same type that was not selected when the Attribute Editor was launched will have the default value of the isManaged (0x1295d) attribute (Maintenance Mode) set to "Yes" but no event will be logged.
Setting the default value in the Attribute Editor changes the default value of the Model Type in the catalog. All existing models of that type currently modeled in the database will inherit the change and all future models of that type created after changing the default value will inherit the change.