Looks like a double wall rim to me. My guess is that if one were to look at the extrusion die there was part of that die that would show why that area could have been a weak spot. Any extrusion die that has a hollow cavity at some point has webbing to place that cavity, the metal flows around the web and under heat and pressure is "welded" back together. My view is a "cold" fusing of the metal happened, and over the years those micro voids became enlarged under pressure from the tire tube and oxidation. Lack of review by the owner and or mechanics let this percolate to the point of failure. Who knows how much pressure the fellow had in the tire too.