3. Primary Osteo

Many factors disrupt the active bone remodeling described in the previous post.  

Muscles attach to bones.  Pull the muscle hard enough and the bone also gets pulled. Specialized nerve receptor cells in the bones recognize these pulls and respond by stimulating osteoclast activity.  Ergo, bones become stronger (more dense) with weight-bearing exercise.  I don't have an answer why inactivity has the reverse effect (loss of bone density) but it does.

Steroids such as prednisone used by asthmatics, or recipients of organ transplants.  These steroids may keep calcium from reaching bones or may disrupt the osteoclast/osteoblast balance.  The body can overproduce it's own steroids, yielding the same results

Excessive alcohol use and smoking are associated with osteoporosis; alcohol because it inhibits calcium and vitamin D absorption and may directly kill osteoblasts; the smoking link is unclear, perhaps just because smokers are more likely to abuse alcohol and to have poor nutrition.

Too much parathyroid hormone, not enough thyroid hormone, any eating disorder (including the simple deficiency of calcium), and certain cancers and renal disease also increase the risk of osteoporosis. Decreased bone density attributed to one or more of these factors is labelled "Secondary Osteoporosis."

I tested negative for all known risk factors, so I have "Primary Osteoporosis," which means an as yet undiscovered factor, or an acceleration of the inevitable bone loss that occurs with aging, has led to my bone loss.