When chains wear, they also get more sideways freedom. Shifting gears to the outside usually happens with a very quick movement, setting up a sideways snakelike wave in the chain. This often has the effect of throwing the chain off the outside.

The cure is to arrest that sideways wave. Usually a chain guard is enough but if that chain guard is too far away from the chain, it won't have the desired effect. The cure is to move the chain guard closer to the chain, perhaps by using smaller spacers or filing the existing ones a bit shorter. Put it as close as possible with the chain on the smallest rear sprocket, without rubbing.