There is a fundamental conflict when growing trees in lawns because their watering needs are polar opposite. Roots follow the water. If you have relied on frequent, shallow lawn-watering and haven't been deeply irrigating your tree for the last 10 years, the tree roots have traveled to the surface to access the water. There is no correcting the situation now, but you can hope to encourage other roots to head downward by properly irrigating your tree going forward (not actually sure it will make much difference at this point).
The two primary disadvantages to shallow roots are tree instability if the soil becomes saturated by heavy rain or flooding, and reduced ability to withstand extended periods of drought.