What causes baby teeth loose?

Naturally: This is preprogrammed in growth and development. As a permanent tooth is growing it will cause the root of the baby tooth to resorb and finally the baby tooth will fall out. Occasionally this does not happen naturally and the baby tooth needs to be helped out.

When permanent teeth: Start erupting, the temporary teeth will start to get loose based on the age of eruption.

You don't: Mother nature does. As permanent teeth develop roots of baby teeth dissolve away and baby tooth falls out. Once baby tooth root has dissolved away enough, and the remaining crown starts to loosen, wiggling the baby tooth may help it fall out. If not sure, ask your very knowledgeable well-trained general dentist, pediatric dental specialist, or orthodontic specialist.