I have significant amounts of fibrosis in the background of the slide of bone marrow tissue from the reaming for a femur nail. what could cause this?

Depends: If the fibrosis is there after the nail has been in place for a while, it may be just natural scarring that the body produces around a foreign body. However, if the fibrosis was found before the nail is in place, it could be a sign of a bone marrow problem; this would likely manifest with other symptoms like anemia or low platelet count. Discuss with your doctor to see the context!