What killed my Dorsett Golden apple? (pictures of roots)

This tree was a little over two years old. About a month or so back I started noticing the growth dying back from the top and I decided to cut into the main leader…completely dead, pulled it up this evening. The root system had these huge knots in it and what looks almost like an arm bone…when I was feeling around the base I found a piece of string that was wrapped around it, probably from when I bought it. wondering if it possibly girdled the tree…thoughts??

on the bright side I have an excuse now to plant another tree…