What causes our baby teeth to fall out?

Baby teeth: the growth and eruption of the adult generally cause the baby teeth to come out. The roots of the baby tooth are resorbed when the permanent tooth is moving thru the bone. In cases where there is not a permanent tooth the baby tooth will remain in position sometimes for life.

Permanent teeth: cause root resorption and baby tooth exfoliates.

Genetic programming. Usually erupting permanent tooth causes root of baby tooth to dissolve away and eventually the crown falls out. But even if permanent tooth is congenitally missing, the baby tooth root will, given enough time, eventually dissolve away on its own. Over-retained baby teeth can cause occlusal problems as their size does not match sizes of permanent teeth. See an Orthodontist.