Too much soil around the base of the tree can compromise the long term health of the tree.
This can be caused by several factors. Many times trees are planted to deeply in the ground. Planting a tree too deep can greatly influence how it grows, and how healthy it will be. Often times bark dust or soil is added around the base of the tree. This can result in excess soil pressing against the lower trunk. Soil and bark dust against the lower portion of the trunk will hold moisture there, and increase the potential for fungus, rot, or water damage. This can travel up the stem, as well as down into the root system and compromise the tree.
Girdling roots is another reason to justify root excavation. Girdling roots is a common issue caused by the containers the trees were grown in forcing the roots to grow improperly.