This just might be a disease like phytophthora or anthracnose, verticillium wilt. To diagnose a disease one needs to know whether it is viral, bacterial or fungal.
Your tree is displaying a disease that might mean taking the entire tree down. Not be replaced by the same species or by species that are susceptible to the disease diagnosed.
This is a good time to get to know your Cooperative Extension Service. Talk to the Master Gardeners and possibly sweet talk them into coming out to your place to get diagnostic samples. The most important place someone needs to inspect is where that dead leader stops and live tissue starts.
We can help but you'll have to get ready to show cross sections of branches, live and dead, leaves, and cambium slices. Do you see any other trees on your property as well as all your neighbor's that shows a large dead branch while the rest of the tree looks healthy?
If you touch that tree with pruners make dang sure you clean them well with alcohol before using those pruners on any other plant.