Too little deep watering, compact soil, or restricted root systems. Lawns are often wrongly watered too frequently and too shallowly. This means the most moisture in the area is near the surface. Tree roots therefore stay near the surface to collect the moisture. Water deeply and less frequently.
Compacted soil is difficult for roots to penetrate.
Taking the path of least resistance, the roots naturally stay near the top of the soil. Not much after the tree has been planted. Before planting, take care to loosen soil compacted by heavy equipment or foot traffic. A high water table allows plenty of water for plant growth but limits soil oxygen. Roots need oxygen to remain alive, and seek it near the surface layers where it is most readily available.