Any idea why my friend's exhaust exploded?

[72timemachine]

Prior to this incident, my friend's 1996 Chevrolet 1500 pickup with a 5.7 liter engine, would be difficult to start. The other day when he tried to start it up, there was a huge explosion which tore open his muffler and spit out chunks of the catalytic converter the size of your fist onto the street below. The muffler burst open at the seam on the driver's side and looked like an open clam shell.

I am no expert but does anyone feel that either the fuel pressure diaphragm could have leaked fuel into the cylinders or possibly one of the poppet valves from the spider stuck open to cause such a powerful after fire?

Thanks for any help.