A crack suggests fatigue, assuming no barrel obstructions, excessive loads or damage.
Perhaps the barrel was made from abnormally low yield strength material. If this were the case, firing the gun with even "typical" powder charges will possibly create stresses above the endurance limit of the material. And after enough cycles, the material develops a fatigue crack.
I suspect though, that the barrel on most black powder firearms is made from mild steel with a minimum yield strength in the neighborhood of 36,000 psi. It's difficult to get steel with yield strengths much less than 36ksi. If, however, the barrel material is supposed to be say 60,000 psi, then I can certainly see where 36,000 psi material could be supplied by mistake. And if the barrel's factor of safety is marginal with 60ksi material, then it will have a short life if 36kis material is used instead.
In order to get to the root cause, I think some $$$ would have to be spent in a lab testing the material and looking at the crack with a scanning electron microscope. Do we have any metallurgists here?