I think probably some of the families don't have a good reason, though many may.
However, I think in a way people are asking the wrong questions with this stuff. The reason that so many people don't comply is that the guidelines and regulations are trying to change human behaviours, particularly their social behaviours which are pretty powerful. And that is not easy.
What's more, it isn't that they are trying to change one thing, or only for a short time. One of the things about behaviour change is that the more you want, the more resources it takes from people, and the longer it goes on, the more draining it is.
You can get over this sometimes by creating new habits, but with covid there are a lot of things working against that. Some of the things being asked are too unnatural or impractical to be adopted long term. The fact that the rules keep changing works against it.
Also, buy-in is important with behaviour change and buy-in is always less when people don't see the rules as likely to be effective, or as inconsistent. Which a lot of them are, for example the idea you can see friends in a bar, but not your mum in your house, is never going to go over. Even if people understand the idea is to keep business operating, they simply won't accept that kind o sacrifice of their most intimate relationships as the corollary.
All of which is to say, people shopping in groups is probably a symptom of the public ability to keep adapting to the rules degenerating as time goes on.