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 2 years, as much of the country has, 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.