Check out this tree I saw in a neighborhood next to ours. They have two like this. It looks like there may have been mulch volcanoes around these trees at one time.

To me it looks like a pot-bound tree planted high that was then mulched? The "collar" that should be a ground level is considerably elevated. Did it grow a new collar? Fortunately the tree seems to be doing okay. What do others think happened here?

The roots tried to grow up into thick mulch to find water. Over the years after layers and layers of mulch, this is what you get.
Plus, yes pot bound plants will do this. Maybe some of both.