The two most common reasons for this problem are 1) Damage from wind, storms, or mechanical means; and 2) Girdled roots. It looks like it's too tall to check for damage, but If you can, check for broken limbs, and just prune them out. Girdling is the result of roots growing around the trunk of the tree, choking it. No nutrients can get up through the trunk, and it starts to die back from the top down.  If it's very severe there's not much you can do about it, but if it's just starting you might be able to save the tree. Here's some information about girdling roots from the Arbor Day Foundation: 
https://arbordayblog.org/treeplanting/girdling-roots-and-what-to-do-about-them/
Hope this helps solve the problem.