My guess is that it has a great deal to do with the exhaust’s path in the canister… including turbulence. My guess is that the internal pathways cause heat and sulpheric exhaust components to concentrate in the spot that ends up being the first breech. That’s only a guess, however.
Stampings cause variations in the thicknesses of the metals too, and that may be the reason. Or perhaps a contributor. Again, just a wild guess on my part.
To really find out would be simple: run a temp map of the canister when fully hot and/or perform a dynamic analysis of its flow and turbulences on a computer model. Of course the program for this probably runs $20,000 or more and the engineering to input the data would probably cost another $10,000. I might even be being very conservative. I’m guessing, of course. And then you’d have to do a separate analysis for every muffler design.
Or it could be random chance. Who knows.