Sounds like your set up is pretty good for avoiding it so I'd clean up pine cones if possible, otherwise I don't see anything obvious that seems like an issue.Run is dry. It’s full sand, but they do go out into the yard which is mixed terrain. Dirt, grass, weeds, plants, pine cones. None of our hens are overweight at all, I keep track to make sure.
What treatment do you follow? Do you always treat?
For me it mostly pops up now and then in my heavier birds. I do allow my flock some access to gravel plus my roosts could probably be lower, and we have a wet climate here with a lot of wood chips in the run, so I just try to stay on top of it. So far I've not had anything worse than medium size bumbles with mild swelling.
I always treat unless it's a super tiny bumble (which often go away by themselves). I soak in epsom salts to soften up the scab, dig it out, flush out the wound as best I can, then vet wrap snugly but not tightly, and soak again daily until I see good clean scabbing and no swelling. As soon as I'm done they're back with the flock. I've not had an issue with birds not healing up from doing it that way.