Certified GKH Gardening Expert
There are several things that could cause this in your tree. If you have had colder than usual winters for the past two years, the stress from the cold may have hurt it. The tree may also have a pest or a fungus. I would recommend treating the remaining tree with neem oil. It will kill both pests and fungus on the tree. You may also want a local tree expert to come out and look at the tree to more accurately diagnosis the problem.