My assumption after years of observation is that there are several prerequisites that promote bumble foot:
breed/genetical predisposition (Marans, Leghorn)
unbalanced diet/vitamin deficiencies (overweight)
height, shape and material of roosts and chicken ladder
type of bedding
soil conditions...
That would be because the reason that initiallly started the bumblefoot would be still in there, as there are often little stones they jumped on or a litlte splinter of glass, metal or wood or some thorns or spines while free ranging.