Bone loss can occur due to a number of factors. If you didn't build bone in your early years (up to age 30), if its in your genes, if you take steriods, ie. for asthma.
You start to lose bone after age 30 and rapidly for the first 5 years after menopause.
That's why its important to maintain what you have though diet and exercise like weight lifting.
All the bodys organs use calcium. When there is not enough it is taken from the bones. That why an adequate supply of calcium is important.
Even if you think you do enough, bone loss can still happen. Don't think that you are immune. If you break a wrist or ankle from a non traumatic incident and you are in your late forties or early 50's, its a red flag, get tested. That's how I found out because I broke my wrist falling off my mtn bike.