Heredity. A genetic predisposition is what leads to the skin condition called psoriaisis. Certain drugs such as lithium can aggravate psoriaisis. In a few cases it can be associated with a strep throat or food allergy. These are triggers which worsen the psoriasis in susceptible individuals.
Autoimmune disease. Unfortunately it is an autoimmune disease manifested by antibodies directed against the skin and underlying structures.