Little leaf Linden
From the picture it could most likely be a root system issue. It appears that the tree has been planted (or originally grown) too deeply as the trunk at ground line looks like a telephone pole going down into the ground. Root systems of trees planted too deeply can result in girdling roots, reduced oxygen for tree roots, excessive moisture held in the root zone, and increased stress on the tree. I would contact a local Certified Arborist to inspect the tree to see if there is something else going on that I am unable to diagnose from the picture. Sadly a tree that appears this far gone with such a sparce canopy this far into the growing season usually means that the tree is on the decline and not salvagable.
You can search for a Certified Arborist by visiting www.treesaregood.org