Which are the causes of Adrenal Insufficiency?

No known causes that I know of. Your adrenal gland is just fatigued and not putting out enough cortisol

Addisons disease (genetics) and too high a dose of steroids given in infection treatment and/or too fast a withdrawal of the same.

According to the NIDDK
Autoimmune disorders cause most cases of Addison’s disease. Infections and medications may also cause the disease.

Autoimmune Disorders
Up to 80 percent of Addison’s disease cases are caused by an autoimmune disorder, which is when the body’s immune system attacks the body’s own cells and organs.2 In autoimmune Addison’s, which mainly occurs in middle-aged females, the immune system gradually destroys the adrenal cortex—the outer layer of the adrenal glands.2

Primary adrenal insufficiency occurs when at least 90 percent of the adrenal cortex has been destroyed.1 As a result, both cortisol and aldosterone are often lacking. Sometimes only the adrenal glands are affected. Sometimes other endocrine glands are affected as well, as in polyendocrine deficiency syndrome.

Polyendocrine deficiency syndrome is classified into type 1 and type 2. Type 1 is inherited and occurs in children. In addition to adrenal insufficiency, these children may have

underactive parathyroid glands, which are four pea-sized glands located on or near the thyroid gland in the neck; they produce a hormone that helps maintain the correct balance of calcium in the body.
slow sexual development.
pernicious anemia, a severe type of anemia; anemia is a condition in which red blood cells are fewer than normal, which means less oxygen is carried to the body’s cells. With most types of anemia, red blood cells are smaller than normal; however, in pernicious anemia, the cells are bigger than normal.
chronic fungal infections.
chronic hepatitis, a liver disease.
Researchers think type 2, which is sometimes called Schmidt’s syndrome, is also inherited. Type 2 usually affects young adults and may include

an underactive thyroid gland, which produces hormones that regulate metabolism
slow sexual development
diabetes, in which a person has high blood glucose, also called high blood sugar or hyperglycemia
vitiligo, a loss of pigment on areas of the skin
Infections
Tuberculosis (TB), an infection that can destroy the adrenal glands, accounts for 10 to 15 percent of Addison’s disease cases in developed countries.1 When primary adrenal insufficiency was first identified by Dr. Thomas Addison in 1849, TB was the most common cause of the disease. As TB treatment improved, the incidence of Addison’s disease due to TB of the adrenal glands greatly decreased. However, recent reports show an increase in Addison’s disease from infections such as TB and cytomegalovirus. Cytomegalovirus is a common virus that does not cause symptoms in healthy people; however, it does affect babies in the womb and people who have a weakened immune system—mostly due to HIV/AIDS.2 Other bacterial infections, such as Neisseria meningitidis, which is a cause of meningitis, and fungal infections can also lead to Addison’s disease.

Other Causes
Less common causes of Addison’s disease are

cancer cells in the adrenal glands
amyloidosis, a serious, though rare, group of diseases that occurs when abnormal proteins, called amyloids, build up in the blood and are deposited in tissues and organs
surgical removal of the adrenal glands
bleeding into the adrenal glands
genetic defects including abnormal adrenal gland development, an inability of the adrenal glands to respond to ACTH, or a defect in adrenal hormone production
medication-related causes, such as from anti-fungal medications and the anesthetic etomidate, which may be used when a person undergoes an emergency intubation—the placement of a flexible, plastic tube through the mouth and into the trachea, or windpipe, to assist with breathing
What causes secondary adrenal insufficiency?
A lack of CRH or ACTH causes secondary adrenal insufficiency. The lack of these hormones in the body can be traced to several possible sources

Many causes are known, however, not all. CAH, Addison's, autoimmune diseases, hypothalamus and HPA failure, pituitary failure, brain damage, diseases like AIDS and HIV, Sheehan's from maternal bleeding, blunt force trauma, hereditary Hemochromatosis with iron deposits, genetics, steroid usage, tumors that can be macro-big or micro-very small, cancer, adrenal gland damage, and many other factors.