What Causes Avascular Neucrosis?

I found out I have Avascular Neucrosis, I m a postal carrier walking all day. I had a fall in 2007 on my rear hitting concrete was given a cortizone injection and steroids before that. Very bad experience with injection my bone was hit. Shortly after I was activated with Air National Gaurd was gone 5 years nothing seem to bother me in that region of the body. Came back in Oct 2012. Oct 2013 had atrip where i felt like I jammed my leg very hard twisted ancle. Doc had me take steroids and physical therapy. Didn t solve issue seem to get worse more than anything. Went back two weeks later he gave me more steroids. Next at appointment Doc said nothing showed on MRI then gave me a cortizone shot in inner thigh area where initial strain was. Got the MRI roughly 6 weeks after I tripped showed 45-50% in right side and 25% in left side. Trying to figure out what may have contributed to this scenario I am in.