If teeth fall out and there is no periodontal disease anymore, how is it possible that bone can still be lost?

Bone loss: Healthy teeth tend to maintain the bone around them. If a healthy tooth is lost, there still tends to be some bone loss over a long period of time as the bone tends to stimulate bone maintenance with function. If the tooth falls out due to disease or infection, there has already been bone loss, even if not noticeable to the patient. Bone may appear greater than it really is due to gum tissue cover

Atrophy: When the bone is internally stimulated by an intact tooth (or implant) during function, the bone tends to remain dense around the tooth (or implant). If the bone has no internal stimulus (pressure) it tends to atrophy and wither away over time. The same principle occurs in other bones of the bony. In essence, the bone exists to support teeth. No teeth, no need for bone.

Stress: Bone exists to buffer against stress. It's a living changing tissue. As an adult you have an entirely new skeleton every 7-8 years. If a tooth is lost the surrounding alveolar (tooth supporting) bone disappears as it no longer has a function.