I've had similar experiences at a casual running club. I moved back to my hometown for 6 months last year and joined a social club that meets once a week. Most of the people there were pretty casual, middle aged and didn't take runnning too seriously. All I wanted to do was have some folks to chat to once a week on one of my easier runs.

However, we get to discussing our upcoming races, previous PRs etc, and the mood shifts when they find out my running times and weekly volume. Nothing super impressive, but obviously much faster than them. It was never the same after that. They'd always snipe with passive aggressive remarks.

I feel like maybe there was a level of insecurity there? That they immediately went on the defensive because they assumed I may be judging them (which I'm not - I don't care how fast/slow anyone else runs)?

All very odd. Very glad to be back at a much more mixed ability club in London now.