From your brief description I really can't tell what may be causing the problem with your magnolia tree.  One possibility is a watering issue.  Magnolias are very sensitive to under and over watering, especially in poorly drained soils.  Other possibilities include a fungal disease, root problem or pests such as scale, aphids of thrips.  (Here is a publication on magnolia scale, something you can look for:  
Magnolia scale is having a good year - MSU Extension)
You might want to consider getting a certified arborist to check your tree and determine an appropriate remedy if necessary.  You can find a certified arborist in your area by going to:  https://www.treesaregood.org