Poor things. Hopefully, it's a height-based issue and not something else in the yard that's the culprit.
Do remember, bumblefoot is essentially a bad splinter or infected cut, so there's no end to possible causes when your birds are free to roam. Out of all my hens, it was only ever the sexlinks that ended up with bumblefoot, so their foraging practices can play a huge part as well. If they tend to be quite overzealous in their scratching and come across some brambles or unearth a bit of glass or broken pottery, that's all it takes.