Klauss, the issue is that - with cable controlled engines - you can get excessive downstream cable loads that can overcome the capabilities of the autothrottle clutch, allowing one throttle to fail to move when the autothrottle commands it. Not an issue if the flight crew is paying attention - they can easily manually move the throttle to compensate. But if they are not paying attention, you can get a considerable thrust asymmetry. If it gets large enough, it can cause the aircraft to roll - again not an issue if the flight crew is paying attention, but if they aren't and the roll becomes excessive it can lead to a loss of control.
That's what happened on the China Southern crash.