Though the photo is not a close-up of the leaves, we think this may be a Japanese maple. Correct? These are tempermental trees and frequently die for no determinable reason. Your tree is 1/3 dead. It appears to be in decline.
We cannot say for sure what is causing the decline. It is not the lichen spots on the tree. We can say that Japanese maples are not full shade trees and your lot looks very shady. It may be getting more and more shady as the years go by.
Here is a good article of causes for tree problems. Run through them and see if any line up with your tree's situation: http://extension.umd.edu/sites/default/files/_images/programs/hgic/Publications/HG201%20Homeowner%20...
Your other option is to have an arborist look at the tree. Get one certified by the International Society of Arboriculture. Many work for tree service companies. You can follow the prompt on this website to find an arborist near you: www.treesaregood.org Many companies do not charge for an arborist visit, just any work you agree to have done.