I have had this happen with two other dogs. When I was in Jr. High, we had a GS that was wonderful. My neighbor and I always took our dogs (her's a collie X) and played in their orchard and up & down the gravel road (we would take them about a mile up the road and tie them to an old motorcycle w/o and engine and tell them to "go home"... what a ride). This road passed between our properties and lead to other houses. One day we were hanging out in the orchard w/ the dogs (they never bothered anyone riding or walking by) and one of our neighbors, about our age, came home from school and had to walk up the gravel road to his house. When Starsky (GS) saw him, he stopped what he was doing and ran straight for him. The kid started running, but obviously not fast enough... he had no recall, he was on a mission... he caught up with him and bit him in the back of the leg, near the butt, then he stopped and came back... mission accomplished. Well, when his parents called we made sure to let him know that we've seen him throw sticks and things at Starsky through the fence (borders the gravel road), when they asked him he admitted it. He obviously did this quite often and I believe that Starsky saw his chance to get the little creep back and did that and nothing more.
I've had Dobies before and love them to death. My last one was one that I got from Animal Control about three years ago. We had him about a year before we sadly gave him up to rescue, because of our neighbor's kids (5&6yrs, that were never supervised, always ran the neighborhood). Any time he heard their voices he went bezerk. If they came to the door to get our kids, he wanted to attack. It got to the point that for his safty and the kids (never bothered any other kids) we sent him to rescue, so they could find him another home away from the 2 monsters. We couldn't deal with what would happen if he got his revenge. And yes, we found out that they had a habit of teasing him through the fence when we let the dogs out to potty or run & play. Our dogs stay in the house 95% of the time and never outside when we're not home.
Sorry, this is sooo long, but just want to let you know that I would take the side of my dog any day when he decides someone's not worth having around.... there's usually a reason and they haven't been wrong yet. They are all individuals and deal with it differently, as you described w/Heidi. Sorry, again for the length.