68 Cutlass 350 4bbl stock. HEI. Headers. Cruising on Garden State doing about 65 going uphill when BAMMMM. Loudest friggen thing I have ever heard. Thought I was dead. Passenger side muffler blew up. Front and rear of it torn like a tin can. True dual, no H pipe or anything, just headers. Headers back was summits 2.5 inch pipes and muffler system. Thoughts??? Did it technically backfire in order for that to happen? to much fuel and going into pipes/muffler and ignited? Car was running fine but now no. Maybe because no back pressure on that side. The ride home was crappy. Sounded great tough lol.
Any thoughts are welcome and appreciated!!!!

 

same thing happened to a guy on here last year w a full size olds. I think the consensus was unburnt fuel pooling in the exhaust

 

This is exactly what usually causes a blown muffler. Often it's caused by a misfiring cylinder that's dumping rich mixture into the exhaust, or even just an overly rich carb. Fuel pools in the muffler and eventually lights off from exhaust heat.