I believe your tree is suffering from Magnolia scale. There have been several reports of it this year. I am giving you a link to our diagnostic page with info about the scale. In that section is another link to an entire publication on how to deal with it. Please read it. http://www.extension.umn.edu/garden/diagnose/plant/deciduous/magnolia/deadbranches.html  One of the suggestions in the publication states recent research results show that soil drenches with the systemic insecticide formulations containing imidacloprid provide a simple and effective approach.