What causes scabs on the scalp?

Healing. Any injury to the scalp will eventually try to heal. Dried blood, mixed with died cells form a layer on the wound surface known as scab. Beneath it a new skin is formed and the scab will eventually fall off the normal skin. It works as a shied, basically.
Infection,growth. Scabs on the head can be caused by a non healing growth such as sun damage or a skin cancer or inflammation cause by seborrhea or psoriasis. It can also be caused by an infection of the hair follicles. An examination will reveal the proper diagnosis and treatment plan.