This picture comes courtesy of Reddit, the poster said his mechanic spotted it during a brake inspection. The reasons for this type of failure are many. My first thought was mechanic damage from a piece of debris caught in the drum. Maybe it was poor metallurgy or heat stress.
The main point of the driver’s story is that he took it to a shop and the technician spotted it before it caused a crash or safety related problem. The most important part of safety is the technician who inspects the vehicle, without them this guy could have been another broken down car on the side of the road.