This is known as lens creep and occurs with many larger zoom lenses. It happens because the weight of the moving parts is greater than the internal resistance in the lens holding it in place. Thus, gravity causes it to extend when pointed downward.
To prevent lens creep, some lenses have a zoom lock feature (such as the SAL-18250). If the lens you are using has a zoom lock feature, make sure it is locked.
NOTE: The lens may also extend and slide out if the camera is mounted to a tripod and a downward shot is arranged by remotely triggering the camera.