Why does excess ADH (vasopressin) get released from the pituitary gland in the hypothalamus when you have lung cancer?

Not from pituitary. Some lung cancers can make adh, (vasopressin) and the inadequate amount of the hormone comes from the lung cancer cells, not from the pituitary gland.
Paraneoplastic. Google "paraneoplastic syndrome" for a detailed answer.
ACTH. Acth not ADH (vasopressin) can be secreted from some types of lung cancer (small cell cancers) and can effect the bodies release of cortisol mimicking a pituitary tumor.
Not so. Anyone telling you otherwise is misinformed. The lung cancer, usually small cell / oat cell, itself produces the adh (vasopressin).