The photos are not ideal for the purpose of diagnosis, but the fracture surface looks like that of a brittle overload failure. That is typified by a fracture surface which looks crystalline, and little evidence of plastic deformation in the fracture. That would lead me to suspect there might be an issue with the barrel, such as incorrect heat treatment, as a potential factor here.
A brittle barrel can survive quite well as long as loads are within its limits, but does not cope well with overload. One should expect that a barrel will bulge rather than crack on an overload - that is a lot safer, as the work done in bulging dissipates but contains the overload, where a brittle failure simply lets go with little work of fracture. Brittleness also does not cope well with stress concentrations such as sharp corners. It could have been as little as a ruptured rim, coupled with brittleness and the stress raiser at the apparent point of crack initiation, and there's your rifle buggered .
If it was me I'd be having a good hard look at hardness and microstructure of the breech end of the barrel. There again, given that it is an older rifle, out of production, and no-one was hurt, it is probably not going to be worth spending the money on any deeper analysis.
