Why do cancer patients often develop endocrine disorders?

Several reasons. As other doctors have noted, radiation can damage glands and reduce their ability to make and release hormones. Chemotherapy can do this as well. Chemotherapy often includes steroids such as Prednisone or Decadron (dexamethasone) which can lead to diabetes and disrupt other hormones. Some cancers make hormones that cause low sodium, high calcium, high cortisol, high red blood cell count, and other problems.
Depends. Can definitively related to the complications of the treatments such as different medications, sit theft and or radiation.
Due to therapy . Sometimes the treatment of non endocrine cancers result in endocrine disorders. Corticosteroids like dexamethasone, which is often given as part if chemotherapy, can cause diabetes and cushing's syndrome. Radiation therapy to the neck can cause thyroid disorders. Some cancers can cause high calcium levels by producing extra vitamin d or other substances. Others can cause low sodium.
Sometimes from XRT. Certain organs and glands are sensitive to radiation treatment, and their function can be impaired if they are near or within a radiated target volume. For example, some patients who receive neck radiotherapy (for cancers of the throat, etc) can occasionally become hypothyroid. This is usually corrected with synthroid (thyroxine) and is easily screened for with serum TSH every 6-12 months (nccn guidelines).