Why do pressure sores sometimes smell bad?

Infected dead tissue. Pressure sores are caused when unrelieved sustained pressure causes tissue compromise or death. Bacteria use the dead tissue as a nutritional source and proliferate. Occasionally maggots may also find this a good source of nutrition. Certain bacteria will produce odors that are offensive. Wound care includes surgical removal of dead tissue to limit bacterial growth.
Bacteria & necrosis. Pressure ulcers are the physical result of injury to the skin. When the injury progresses to an open sore, the sore will be invaded by bacteria and can become infected. The buildup of bacteria in the wound can cause odor. Also along with bacteria, the injured tissue can rot and this produces odor, similar to leftovers that have spoiled in your refrigerator.