My #1 most likely theory: Sabot from previous shot hung up in the muzzle-brake creating a barrel obstruction.
Some of the new black powder substitutes like Buckhorn 209 don't give the big cloud of smoke like we're used to seeing.
I haven't tried, nor would I, nor should anyone else, but, I don't believe you can actually "overcharge" blackpowder and get that sort of a reaction so long as you've compressed the charge of loose or seated the bullet up against the pellets. Blackpowder is strange stuff compared to smokeless. Blackpowder is explosive when loose, however, when compressed it is a propellant with a consistent burn rate. Give blackpowder some space however and it = KABOOM! A projectile seated say 1/2-2/3 of the way down the bore rather than tight to the powder charge could also cause the KABOOM from above.
The reason I don't believe the rifle was over-charged with powder is that every now and then you hear about a person forgetting the ram-rod and shooting the thing off to kingdom come. The ram-rod weighs 5-10x (or more) what a normal bullet does and it doesn't blow up the rifle.
Edited to add: ALWAYS put a mark on your ram-rod that equates to a fully compressed powder/bullet charge. You will shoot more accurately and will avoid things like a double-charge.