For what it's worth, MODERN revolvers have been blown up with reloading mistakes with results closely resembling that of the Remington in the link.
I've seen photos of Ruger Blackhawks that blew up in exactly the same way. My guess is a double charge -- I don't buy the double bullet theory.
Previous explanations about diameter are correct. I used to have one of those .44 caliber 1858 Remington clones. The instructions with mine called for .454" lead balls. The bullet diameter for .45 Colt is .452". The conversions work Very well -- I've shot both the R&D type that was "featured" in this incident, as well as the competing Kirst conversion. It's just unfortunate that the conversion cylinders cost more than the guns.