Necrosis is a generic term for tissue death. Therefore, for example, you can get skin necrosis, fat necrosis, bone necrosis etc. The common underlying mechanism is loss of blood supply. What causes that loss of blood supply? Surgical causes include cutting skin/tissue too thinly, fashioning a strip of skin/tissue that's too long and narrow, cutting a critical artery that supplies an area of skin/tissue, closing a wound under excessive tension to name a few. Medical conditions that can contribute to this in patients undergoing surgery include diabetes and vascular disease because of their effects on reducing the circulation in tissues. Infections can contribute to skin/tissue necrosis, so not keeping your incision site clean may be an indirect cause.
Skin necrosis can occur for many different reasons. One reason is that the blood supply to the tissue is not adequate.