I'd be really interested to know which festival it was. Not for salacious reasons, but just because I have a theory that some festivals are more likely, because of lineup and target audience, to attract this sort of behaviour...

I also suspect I may know the culprits. Was it self-penned stuff? Mostly guitars? Or lots of boxes and suchlike?

I think what happens (and this does NOT excuse the rudeness you experienced) is that certain groups of young players meet at a series of festivals each summer - they know each other, they know each other's playing, they like playing together. It can feel a bit exclusive, to be honest. At festivals where there are a number of sessions, that doesn't really matter, because you can go and find another group to play with and leave them to it. But at a festival which doesn't feature many alternatives, I imagine it can be off-putting, as they seem to dominate things.

Just my observation. As i say, it sounds like they were horrid to you, whoever they were.