Suntiger,
There is an article written about how they occur,
http://www.geojohn.org/BlackPowder/bps1.html (a whole series of good reads).
I also read on a forum where a guy had a brass framed Colt, with the Ring on the front of the frame all hammered up from the cylinder slamming into it, caused number 2, 3 & 4 chambers to go off when he fired it. He showed pictures where you could see the capped nipples hit the frame because the Ring no longer supported the cylinder. He then removed the Ring, soldered in a washer of some sort (which I thought looked like a good fix) then decided to scrap out the revolver. This was caused by overloading the brass frame revolver.
I think the article by John Fuhring makes a good argument for combustible cartridges.