What are the reasons for painful crises in sickle cell anemia ?

With worsening: "sickling" the blood supply to tissues and organs is decreased. This can cause bone pain, acute lung changes and other serious manifestations of sickle crisis

Vasoocclusive event: Episodes of pain are the most common type of vasoocclusive event. Pain may be precipitated by events such as weather conditions, e,g,, high wind speed, low humidity, dehydration, infection, stress, menses, alcohol consumption, and nocturnal hypoxemia. However, the majority of painful episodes have no identifiable cause.