Something may have eaten the roots or be affecting the plants water system in some way.Or it could be a fungal infection. Verticillium wilt can affect Magnolias, this is a soil borne fungus that causes the leaves to wilt and die back and also affects branches. There is no cure and the affected tree must be removed with as much root as possible and don't plant a tree in the same spot for a few years. This all sounds very drastic so I hope that it isn't this fungus. Unless of course it's in a pot? But that would be hoping for too much! Sorry.

If it is in a pot then vine weevils may have chomped on the roots..