Causes of boil on the buttock?

Depends. These often arise when germs enter the hair glands at their base and fester or germs planted from scratching. Those near the anus may arise from a fistula inside the large intestine. Aggressive boils suggest you have a resident colony of staph in your nose.

If by boil, you're describing an abscess or similar skin infection. Usually they start as folliculitis, an infected skin follicle, which later turns into a pustule/white-head. If it gets worse, a "boil" forms involving deeper skin layers. The most common bacteria are the same ones that live on your skin surface (staphylococcus and streptococcus), then enter through a cut or irritated hair