Looks like there are some scattered remnants of foliage - possibly an arborvitae or leyland cypress? (I'm not familiar with the latter)

Looks like fungus to me.

-Babka

That does look like a pot. wow

Is it a pot or a drip irrigation line? Regardless, remarkable picture!

Is it possible that this could also be a rare case of graft failure in an older tree? The break is pretty clean. See the bottom picture here: https://propg.ifas.ufl.edu/06-grafting/01-terms/03-graftingterms-incompatibility.html

It could very well be girdling though as you can see the one distinct poor major root. Too bad there isn't a third pic showing some excavation. I'd love to yank that stump out with a skid loader and see what the rest of that root mass looks like.

Thanks for posting.