Well I'm no expert but since everyone is speculating........
If it was an explosion, however caused, the entire muffler would look more like a balloon or a 2 litre pop bottle. It looks perfect aside from the hole. When my toolbox exploded, it became round despite the many areas for the pressure to escape (no further comment
). It's all about the speed of expansion vs. the area of resistance.
In my experience, the metal/carbon fibre whatever construction develops fatige cracking from vibrations/resonance and the crack progresses until failure. Titanium jet exhausts are extremely resistant to heat but crack all the time from thermal expansion and vibration. Hell, there's a whole shop dedicated to exhaust repairs in most Airlines. Just another day.
What do I know?