Your walnut leaves are falling from a little insect called a butternut curculio.  If you look at the petiole end of the leaf where it was attached to the tree, you will notice that it is dark and broken.  That's the trademark work of this insect.

There is nothing you can do to stop this.  The damage is done and the leaves that are dark and broken will fall.  It will not kill the trees or even damage them.  I have never heard or read of a treatment but it is difficult to spray large walnut trees and know how much in advance that eggs are laid. 

If you are worried about the trees, water them.  It has been extremely dry this year.  Keep watering them so they can grow new leaves.