I live in the south east and the back portion of our yard is populated with very tall (40-50 ft) long leaf pines.
Our 6 girls have access to the whole yard.
We've had a family of squirrels who have been FEASTING on the large immature pine cones growing way up high for the past 3-4 weeks.
They sit way up there peeling each "petal" away from the cone, eat the tiny nut inside, and then let the petal fall to the ground. Each one of these has a small needle sharp protrusion on it. There are many hundreds maybe thousands of them falling down to the pine straw " floor" .
Impossible to eliminate without cutting down trees, or getting rid of the squirrels. I'm doing what I can to rake the litter.
2 of our 6 have minor cases of bumblefoot which I'm currently treating non-surgically.
I don't know for sure but I'm thinking that's the source of our problem.
Interesting note though, is that both girls who have it are welsummers. None of the buff orpingtons or the australorp have it!