In my experience with friends and family experiencing road rage from other drivers, they usually deserve it...ie:they are crappy drivers.
Example:I have a friend that when stuck in highway traffic drove for miles on the shoulder to get ahead of everyone. She did this on the way home from Wisconsin with the mini van full of children and in front of everyone, while she was doing it, asked me if I thought it was wrong for her to do. Of course it was wrong...it was even more wrong in front of the kids, teaching them that this was an ok thing to do! I have driven with her so many times and she drives like a nut case. She complaines all the time about people shouting things at her while she's driving...I'm sure she deserves it.
I have another friend that complains all the time about people cutting him off on the road and yelling at him or "starting something" with him while he's driving. I have driven with him many times and he doesn't use his turn signal and drives erratically. He drives like a nut and is always swearing at other drivers. I finally told him that I won't drive with him anymore because it is always such a disturbing experience. He deserves the road rage.
I could go on but won't. I'm getting pissed off just thinking about it...so not worth it.
So in conclusion. The people who experience the most road rage coming from others probably deserve it.
In your case...maybe mom and daughter just had a huge fight and sent it out the window at your husband. Maybe they were both texting someone and looked up and saw your husband backing up and freaked out...who knows?
Be honest with your kid...he needs to start learning what driving is all about!