Frequently, bumblefoot lesions will develop when the bird steps on something sharp, allowing bacteria to penetrate inside the tissue, which oftentimes will become infected. There are various other causes, such as rough grounds, and nutritional deficits (such as vitamin A) that can predispose a bird to developed bumblefoot also.
If one starts to see this with frequency, there is likely some issue occurring that can be fixed; always strive for clean, soft, dry grounds, as well as feed a diet nutritionally fulfilling to the birds.
Treatment for bumblefoot will often entail soaking the foot in antiseptic water to help disinfect any superficial contaminants; surgery will sometimes be indicated where topical treatment would otherwise be considered ineffective. There are various links online with more in-depth information.