A deficiency in the release of vasopressin (antidiuretic hormone) by the posterior pituitary gland. Why?

Possibly infarction. Possibly a tumor or something else. Lack of blood supply to the pituitary gland or a pituitary tumor may change the secretions of adh (vasopressin). However, there are other causes also, such as pregnancy that can cause this area to be infarcted (loss of blood supply).
Different things. Brain tumors, some brain injuries, congenital brain anomalies ( like septooptic dysplasia). Anything that causes damage to the posterior pituitary gland or its development can lead to diabetes insipidus (di).