As others have mentioned, the blood supply to your skin (subdermal plexus) was compromised by any of a number of possible factors. If you smoke or have a history of smoking, then the delicate microcirculation that supplies blood to your skin could have been impaired. Raising a skin flap in a plane that is too thin, aggressively dissecting too large a flap, excess tension on the wound, excessive compression from dressings, untreated hematoma, etc. can all lead to vascular compromise and skin flap necrosis.
Some treatment options to consider with your surgeon: hyperbaric oxygen, Trental (medication to improve bloodflow), and Santyl ointment (to help the necrotic areas heal more quickly).