For some schematic datasets that contain an important number of data sources (such as schematic datasets working with ArcSDE data shared by several users), opening all data sources can be very costly in time. For this reason, the data sources referenced in a schematic dataset are never automatically opened by default when the schematic dataset is edited in the Schematic Dataset Editor.
On the other hand, you can enable the Open data source connections running mode if you want them to be automatically opened when expanding the Data Sources branch in the Schematic Dataset Editor tree (Tree view) or clicking the List Data Sources item (Details view). To enable this running mode, you must click the Preferences drop-down menu on the toolbar and check the Open data source connections menu item.