When a root closely circles a tree trunk rather than growing outward, it is called a “girdling root”. The situation usually starts when the tree is young. Somehow a root begins circling the trunk. As it grows larger and the trunk expands, pressure against the tree base limits water and nutrients going to the leaves.

I see this frequently, especially on maple trees that were grown in containers. If less than one-fourth of the trunk is damaged, remove the root now. Wait until winter to remove larger roots.

Details at  Removing Circling Roots

circling root will damage main root

girdling root

tree flare roots growing outward