The shift process sends a wave down the chain, similar to what you do when you try to flip a snagged garden hose or rope over an obstruction. If the chain is brushing the outer ring, the wave can "bounce" off the ring and push the chain over, or a vertical wave interferes with the one tooth at a time engagement on the inner ring, allowing the chain to snag on a tooth, ride the tip and then derail.

You can add a chain watcher and maybe should, but the key is to address what causes the shadow by separating the rings, or moving the crank out, whichever is practical.