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