my 40+yrs experience as shooter, hunter, gamewarden, hunterEd instructor says that it's 10 to 1 odds it was a bore obstruction.
I've seen dirt daubber nests, mud plugs from sticking bore in mud, rats's nests (12ga shot gun), cigarette butts, bullets stuck in bore, .22lr dropped in .30cal bore, 20ga in 12ga, .38spl in bore of .41mag, you name it....... Even an 8mm Mauser in .30/06....not pretty btw.
My bet is bore obstruction.......
Although it's remotely possible that a defective load (very unlikely if factory ammo) or poorly stored ammo that had deterioated. I had some H450 go bad and an "accuracy" load from a .30/06 w/180gr bullet locked up the bolt and removed case looked like a belted magnum...... but gun was unharmed.... primer was blown however and pitted the bolt face...... I pulled down the rest of the ammo and powder "stunk" like vinegar and inside of cases were turning green from acid corrosion..... Ammo had only been loaded 6mos earlier... and powder looked and smelled "ok" then.
A storage room that is un-airconditioned or if stored in a vehicle, temps can exceed 140degF. and deterioate ammo.
Hence the admonition to "store in a cool, dry place".....
Glad you weren't hurt...