There are many causes of Avascular Necrosis (AVN)

Anything that hampers the blood supply can leads to AVN.
The three main causes are:
1) Excessive use of Steroid
2) Alcoholism and
3) Traumatic Injury
Other than that, Hypertension (high blood pressure), Thrombosis (blood clot) and Sedentary Lifestyle are also considered as the cause of AVN.
However, risk factors also include: Caisson Disease (decompression sickness), Vascular Compression, Vasculitis, Arterial Embolism, Damage from Radiation, Sickle Cell Anaemia, Gaucher’s Disease, In some cases it is idiopathic (no cause is found), Rheumatoid Arthritis and Lupus are also common causes of AVN