My muffler exploded! Has anyone else had this happen? What was the cause?

I was starting up my little truck when suddenly, KABOOM! and it started like normal, just really loud. I looked underneath to see that my muffler was laid open like a bomb had gone off inside. The truck runs fine and I am going to the muffler shop for (I hope) an free new muffler. Has any other fluther had an IED go off in their muffler? Were they able to determine the cause of the explosion?