Could be the limbs had weakened attachments to the trunk. I suggest you have an onsite inspection from a trained arborist - a tree doctor (you can find one here: http://www.isa-arbor.com/faca/findArborist.aspx). Your tree may need pruning and could sustain deadly damage in a storm. Also, falling limbs can injure and kill. Best and Happy Yardening, Nancy
Apparently Healthy Southern Magnolia Tree Is Dropping Limbs
Question From: T. Graves - CALIFORNIA
Q: We have a 20-25 foot southern Magnolia in our yard in Northern San Joaquin county, CA. This year it has dropped 2 large healthy limbs from fairly high up on the tree, leaving significant holes in the foliage. What can cause this and what can I do about it? The tree looks healthy in every other way.