I'm now convinced.
When my friend was over and chased Bridget, I noticed that for the next day or two Rose chased Bridget (and was possibly the cause of her leg injury). When I chased Rose for eating the black walnut leaves, George lunged at her and bit her.
And since the colic incident, George has been very aggressive to Bridget. In my anxiety to prevent her lying down and rolling, I was very forceful toward her - smacking the ground with the rope when she lay down, and twirling the rope at her to keep her walking if she started to show signs of going down. I think he thinks Bridget is fair game now.
I'm convinced now it's no coincidence. I'm sure that they copy us - making it all the more important that we mind our manners, stay tactful, always say please and thank you, and generally model the kind of behavior that we'd like to see from them.