The fast rate at which this dogwood is dying suggests that something changed suddenly in its environment. Was any kind of digging/edging done in the area? Pesticide/herbicides applied to soil or other plants, either granular or spray? Mulch applied? Soil put down? Was the soil grade changed anywhere in the vicinity so that it gets more water? Does water sit for long periods on the area?
Keep in mind that the roots of trees can extend 1 1/2 times the height of the tree.