It is in a constant process of remodeling. Bone tears itself down and builds itself up using special groups of cells that are like construction crews. One crew is responsible for tearing down the bone and the other crew is responsible for building it up again. Normally, that is a balanced process and your bone actually rebuilds a new skeleton every 10 years. When it becomes out of balance, then osteoporosis develops.
Normal bone is like a really strong scaffold.
It has organized supports all along its structure. When you start leaving out some of those supports, the scaffold gets thin and obviously weaker. Osteoporotic bone has lost some of its inner supports when the remodeling process is out of balance. This makes it much easier to break down and collapse. When you use your muscles and bones, it stimulates the building crew to work harder and faster.