When the mouse pointer is in the Graphics Window, clicking and holding the left mouse button as you drag will perform the current transformation (for example, rotate or zoom) as selected under the Graphics Window transforms icon in the Transformation Control area.

To perform a different transformation, you have to click on this icon and select the new operation.

To avoid this, you can redefine how the left mouse works as well as map additional transformation operations onto the middle and right mouse buttons, combinations of mouse buttons, and double-clicking of mouse buttons.

This customization only affects the mouse usage while in the Graphics Window. The left button is still used for other user-interface actions.