I think we have a girdling root problem here…
Roots can become wrapped around the main trunk of the tree, choking out water and nutrients from moving up and down the tree. The tree will generally become stunted on the side of the girdle, slowly showing signs of decline. This problem is often compounded when other biotic or abiotic issues are present. Some trees are more prone to girdling roots than others.
This is a slow death for the tree, often going unnoticed by the owner. At the stage you see here, help is still possible. The crown of this honey locust shows minimal dieback and removing the girdle could be helpful.
On bigger mature trees, root pruning can be much more costly and time consuming. Sometimes it may even be to late to worry about. Sometimes the damage you may inflict in removing the root could be worse than leaving it alone. If it isn’t too late, an experienced certified arborist armed with an airspade, air compressor, and a few pruning tools should be able to help the tree breath a little better.
This root problem is best solved at time of planting though. Make sure you look for this sort of thing at time of planting or when you buy the tree in the first place. You should always be able to see the root flare at the base of the tree. If you can’t see a flare you may have a problem. A little root pruning at the early stages of a trees life can easily correct this sort of thing.