My potatoes all did something similar this year - they died back to the ground, and when I dug to look at the roots, there was nothing to find, like they had completely rotted away. I'm not sure what caused it either - we've had a strange year for weather, and I also know that I have root knot nematodes in the soil (I had suspected them initially, but without knotty roots, I don't think I can really blame them).