“What is the cause of osteoporosis?”

Typically age related loss of bone strength.

Primary mechanism of osteoporosis is imbalance between bone breakdown and bone deposition in your body. Bone is a live organ. There are cells that constantly break it down and other cells that constantly lay done new bone. When there is imbalance between how fast these 2 types of cells are working, bone gets broken down faster than new bone is deposited which results in thinning of the bone which is called osteoporosis. In women, this is something that happens universally when they go through menopause and the bone density decreases rapidly. That is the "main cause" of osteoporosis in women.

Aging, combination of genetics and inadequate Calcium/Vit D

Your diet is the key. What bone needs to do right are Calcium Phosphate (cheese), Vitamin K2 (in butter), and Vitamin D (over-the-counter 1000-2000 IU in gel form). They need to be taken together. D vitamin is fat soluble, means if you do not take in with cheese, butter, or eggs at the same time, will never get absorbed.