Sudden movement can effect the auto focus mechanism similarly.
Focus Tip -- Lets assume you are at the back of an auditorium, video taping a singer and some performers. With your camera on a tripod, select "Manual" focus. Now zoom in as tightly as possible on the stage. Select an object on the stage and focus on it carefully. You should now be able to zoom in and out freely, and the camera will remain in focus throughout the zoom. If you change the position of the tripod, you will have to repeat this procedure.