Such cannons were offered for sale years ago by places such as Dixie Gun Works. They were intended for black powder use only. As with any black powder replica gun, only black powder can be used. Using modern smokeless will cause the firearm to explode.
I suspect the idiots in question used the wrong powder, and too much of it.
My shooting experience with black powder is you cannot overcharge a weapon. Excess powder simply blows out the muzzle before it burns as it burns too slow, which is why it’s safe.
This is very sad. Explosives are not toys, and the destructive capability of such requires extreme care and respect, and is best avoided altogether. It is interesting that there was an episode of Columbo (”By Dawn’s Early Light”) based on this exact premise, where a ceremonial cannon is secretly loaded with a more powerful explosive than usual, so the cannon explodes, killing the intended victim in an apparent ‘accident.’