Lots of brainstorming over the weekend, and all the answers were legitimate. A few people came close with the observation that the roots looked like they had grown over something. And that’s exactly right:
This is a great example of nurse log decomposition. When the tree on the right first began growing (and it could have been decades ago), it sent lateral roots out, over, and around the nurse log to reach the soil. As the nurse log degraded, the tree’s roots were left high and dry, outlining the girth of the original log.
Does this natural example have application in managed landscapes? Absolutely! As several of you pointed out, removal of soil or organic matter by erosion or decomposition can leave woody roots exposed. If these roots are injured by feet or tools, they can lose their bark and become open to disease or pests. These are the structural roots of the tree, and if their stability is compromised, so is that of the tree.
(Though this tree has had some injury to its roots (probably from hikers), it’s unlikely to fail as it’s pretty small. )