Genetical : It runs in family.
Overactiveness and malfunctioning of immune system : Psoriasis is caused due to overactiveness of immune ststem. Human body has natural in built mechanism (immune system) to protect itself from foreign bodies like fungal spores, bacterias and viruses. Due to mistaken Identity T-Lymphocytes of immune system start attacking healthy skin cells considering it as foreign objects. As a result of that basal epidermal skin cells start proliferating around 10 times the normal speed. Immature keratinocytes piles up on skin surface as skin shedding/sloughing stops. This is how thick skin palques are formed in Psoriasis.
Psoriasis is an autoimmune skin disease characterized by T-cell mediated hyperproliferation of keratinocytes.