Ischemia. Blood circulation is essential to keep tissues alive. When pressure causes decreased blood circulation, the tissues die and create a hole (ulcer).
Blood flow pressure. As it relates to decubitus ulcers, enough pressure has occurred over the area in which the wound is located that the capillary blood flow has been occluded. Not only does this occur, but the occlusion of the microscopic blood vessels has to be for a long enough time that the oxygen supply to that area is cut off long enough to allow tissue to be harmed. There are more technical explanations.