Smokeless powder grains are by weight. Black powder grains are measured by volume. I would surmise that the shooter measured smokeless powder (weight) with a BP powder measure (volume). Thence boom.
This isn't really reloading, but I guess you can argue that all muzzleloader shooters are indeed handloaders. Anyway, I've been bang-flopping deer with my Savage 10ML-II for 5 years, and have no reservations or concerns about continuing to doing so (yes, with smokeless powder). There have been several incidents of failures, and it seems like most have been in poorly maintained guns (barrel/breech plug erosion) or questionable/unsafe practices (not re-seating a load after failed ignition).
My pet load (a 300grain bullet at 2275fps) should produce a "chamber" pressure just a touch over 30,000psi. These guns are proofed at 129,000 psi. I'm much more willing to shoot my Savage (with smokeless) than some of the other pipe-bombs-with-a-stock muzzleloaders on the market.
For those who really care about this story, read this:
How to Blow Up a Savage 10ML-II
Good read!