It may be my imagination, but a few of the branches look like they might be pinned under the wire in a way that would cause them to die off. Improper wiring technique may be part of the reason you are losing some branches and not others. Another possibility is that the tree is shedding foliage that was being supported by the particular roots you cut, especially if you cut too many small "feeder" roots and left the larger roots in place.
You were also under-watering it, as others have pointed out. Depending on how hot your summer has been, that might have shown up already, or it might take a while longer. However, the tree does look alive to me, on account of the new buds...