The default center of rotation is defined as a point approximately in the center of the geometry.
In the Contextual mouse menu (the menu which appears when you right-click over the graphical window) the option 'Automatic rotation center' / 'No automatic rotation center' is listed. If this option is active, for each 'Zoom In' / 'Zoom Out' / 'Pan' the point of the geometry or mesh nearest to the center of the screen will be selected as the center of rotation for subsequent rotations. This variable is also present in the Right buttons menu under Utilities -> Variables.
If a new Rotation center is selected, this option is deactivated.