This is a rampant problem and has been for nearly 20 years. When trees are planted too deep, the roots suffocate because they don't get enough oxygen and eventually the tree dies.
Planting too deep also can cause the roots to girdle, or wrap around the tree, cutting the flow of water and nutrients, which also causes death. And, too-deep planting puts the tree trunk in contact with soil, which can cause the trunk to rot.
Part of the problem is that in too many instances, trees coming to local landscape companies from wholesale nurseries already are in a flawed condition. That is, they are too deep in the container or too deep in the burlapped ball. If this situation is not corrected at the time of planting, the tree is doomed.
Whether you are planting a tree yourself or hiring a landscape company to do it for you, you must check to see that the root flare at the base of the trunk is visible above the surface of the ground.
Do not assume that the landscaper knows this.