ORIGINAL: Underclocked
No, I think it had to be something far worse than a short seated bullet. Maybe one load on top the other...
The only picture I've seen of the rifle was so bad there was no way I could even tell what I was really looking at.
This probably happens more than one would thing. The shooter next to me was shooting his brand new inline last month prior to the muzzle loader season and he double loaded. Fortunately he double checked against his witness mark on the ram rod and realized his mistake before dropping the hammer.
It is very easy to get distracted at the range and make a mistake.
Bob