According to SkinPick.com, people pick and eat their own scabs because they have a skin picking disorder or compulsion. It is considered a psychological disorder in the same category as other self-harming behaviors.
Picking and eating scabs is a disorder that falls under the heading of excoriation or dermatillomania, Wikipedia explains. Those with this order pick ritualistically and obsessively at their skin, usually at a place where there is a blemish or a scab. This behavior is accompanied by anxiety or stress, and doing it affords the person a certain amount of relief. One person who suffers from this told ABC News that there is pleasure in the picking.
Trichotillomania Learning Center says that skin picking can be symptomatic of certain health problems, such as autoimmune problems, psychosis and obsessive-compulsive, developmental or substance abuse disorders. Although it is still a mystery why exactly some people have this problem, it is often triggered by a disease or injury that results in a healing wound that can cause the person to pick at it. Another trigger for picking and eating scabs is a very stressful event. Although eating scabs is not harmful, obsessively picking at the skin can cause permanent discoloration and scarring.