Hidradenitis suppurativa develops when the oil (sebaceous) glands and hair follicle openings become blocked with fluid, dead skin cells and other matter released from surrounding apocrine sweat glands. When these substances mix with the oil from the sebaceous glands, they can become trapped and push into surrounding tissue. Bacteria can then trigger infection and inflammation.
It's not known why this blockage occurs, but a number of factors - including hormones, genetics, cigarette smoking and excess weight - may all play a role.