Looking at your pictures, it is hard to tell if there is much of a flare at the base of the tree- a buttress.  If it goes into the ground fairly straight, it would mean the tree was planted too deep, and it is at an age where it now is starting to suffer stress from this.

Look and see if anywhere near the bottom of the tree (maybe from the base up to 2 feet or more) looks flattened and not rounded.  This would be a sign of a girdling root, although this sign is not always visible.  Trees can slowly weaken and die over a period of years or decades because of root girdling. Roots begin to grow around the main stem of the tree and cut off or restrict the movement of water, plant nutrients and stored food reserves.Over time, growth of the branches on the side of the plant affected by the girdling will be slowed. As injury progresses, leaves will become smaller and lighter green, fewer leaves will be produced, and eventually the branch will begin to die back. Death of the entire plant can occur in five to 20 years; watering, fertilizing and pruning will do little to correct the problem. Lindens are one of the trees prone to this problem.I gave you some other possibilties in another answer just a while ago.  But from the pictures, girdling roots appears as a potential problem. There are websites that describe this. A short, easy to understand info sheet is from Ohio State: http://ohioline.osu.edu/hyg-fact/1000/1139.html