Canklecat may be on to something. I probably heard left, instinctively looked left and moved left. In my defense, I was on a sidewalk and would have stepped into the street if I had moved to the other left
. Operator error on my part. I usually use the phrase "rider back" and wait for everyone to settle down. A bell would work just as well, I expect, although I probably would have jumped straight up! Ironic that I was passing under the bike trail bridge at that moment.