This is what I wrote after a little contemplation and study...
I've been loading and reloading since I was 16, and at age 59, I have a bit of experience under my belt, along with a degree in engineering. With that in mind, I've been thinking and talking to others about this, and there are several possible causes. The first possible cause is an accidental over charge of powder. I have a powder check die on both of my Dillon 650XLs, but a distraction could have caused me to miss the high pitched electronic tone. When in our modern lives are we 100% free of distraction? The barrel was clear, both before and after the shot, so obstruction is out as the factor. Another cause could be metal failure in the conversion cylinder; it was the first round that was fired through it. The cylinder is made from 4140 steel, and the cap assembly is made from 4150, and they are proof tested to SAAMI standards for standard velocity ammunition. Another is the curious, but not well defined, occurrence of a small powder charge being ignited in a large cartridge case, somehow causing a detonation, rather than the normal progressive burning of smokeless powder; blowing up the piece. Many respected gun writers, from Charles "Skeeter" Skelton to Mike Venturino, have tried to come up with an answer for this, and failed. This remains an unsubstantiated event, and should only be considered when all other possible causes are relegated to the trash bin. To the double bullet theory; I have to feed my bullets one at a time, by hand, in my 650XL progressive reloading machine, and for one cartridge to end up with two bullets would mean an empty case goes by the crimp die and falls into the tray, scattering powder everywhere; that did not happen. The bullets that I use come from Bear Creek Supply and are molybdenum disulphide coated, so there is no wax lube to build up to trap a slug. I spoke to the folks at Taylor's today and the conversion cylinders are not warrantied when used with reloaded ammunition, which if you read the fine print, you will find in the flyers included with guns from many major makers. Also, Taylor's does proof the cylinders with SAAMI .45 Colt proof loads, so the misconception that these cylinders are only safe with black powder it just that, a misconception. Smokeless powder is built on two compounds, nitrocellulose and nitroglycerine. Some powders use one compound and are called "single based", others use both compounds and are called "double based". Smokeless powders has been around for well over a century and is safe when used properly. Abandoning smokeless powder because nitroglycerine is explosive, in the pure state, is simply silly. We might as well give up gasoline powered automobiles, as gasoline is flammable in the liquid state, and explosive as a vapor. Black powder is an explosive, and cannot be shipped in any way other than by truck. I have seen other guns blown up in my time. They were all muzzeloaders, using black powder. Some guys just think that a gun barrel can be filled with BP, a ball seated (but not completely on the powder), and that it's all good. Nothing can be more wrong. This leads to split barrels and chunks of the breech flying in all directions. I was well along to becoming a member of the American Mountain Men, the hard corps pre-1840 brigade, until I was told I'd have to lose my big safety eyeglasses and trade for watch crystal versions.
When it comes down to it, the most likely cause, by Occam's law (that being the simplest explanation will be the most plausible until evidence is presented to prove it false) is that somewhere I screwed up; operator or "Pilot" error. Anyway it goes, I'm out a pistol and cylinder, which is trivial compared to losing an eye or having to have my trigger finger sewn back on. I'll take Harry Truman's position and say, "the buck stops here", with me, and be glad there was nothing wounded other than replaceable steel.
But how else am I going to get that projectile to go 1300+ fps? 
