Did they ream out the chamber at the same time they altered the forcing cone area or is that a normally thin area that just gave it up over time? I think that is a thin area that got a small crack in it, then got weaker over time as the crack got longer or the entire chamber area became separated.

I had a Fox 20 which had a small split area on the left barrel that vented into the space between the ribs. Barrels had been cut and they never sealed the end back up so moisture got into the area between the ribs. Over time it became a split that vented gasses. I noticed it when I was going to fill in the space between the barrels for a rust rebluing. 20" from the chamber area it made no real difference to me. It was ported into the space between the barrels and ribs. Had that been in between the chamber I could see it blowing up just like this gun did. Rust is a very bad thing for guns.
.