Teeth are anchored into the jaw by long roots. The roots of most teeth are 2-3 times longer than the actual tooth itself. Furthermore, some teeth (molars) have multiple roots, which provide for even more anchorage. Here’s an interesting fact: top molars have 3 roots, while bottom molars have 2 roots. Incisors and canines only have one root.
When permanent tooth buds start to grow larger in size, they propel towards the surface. This migration towards the surface puts pressure on the roots of the baby teeth, causing the roots to shorten and resorb. As the permanent tooth gets closer and closer to the surface, the root of the baby tooth will get shorter, which makes the baby tooth get progressively looser. A baby tooth will only get loose if a permanent tooth is ready to grow in.
When the time comes for the baby tooth to fall out, the root of the baby tooth will be fully resorbed. That is why when the tooth falls out, we never see any remaining root fragments.
The main exception to this rule is if a tooth gets traumatized from a fall or accident. This blunt force can cause the loosening of teeth. Depending on the severity of the injury, often the loose tooth will tighten back up, or in other instances, the blunt force from trauma may cause the tooth to fall out earlier than expected.