Like a few other have mentioned, if the problem had been a obstructed barrel then the barrel itself would have split. Also its very unlikely barrel was obstructed if the previous round fired ok.
That leaves a ammo mixup or a previously damaged action.
I took a guy elk hunting last year that missed two 75yd shots at a elk, shooting from a rest on a log. Both shots hit way high as I saw the 2nd one hit the mountainside behind the elk. Found out he was shooting 7mag ammo in his 300mag. Easy mixup as when he grabbed 4 boxes of Federal Fusion ammo off the shelf there was one box of 7mag mixed it and he never looked close, then the morning we went hunting he grabbed the wrong box. He and another friend found the mixup when going out the next day to check sight in and he shot a couple more before the mistake was discovered.
I can see where different ammo could easily get mixed up in the same box either intentionally or by accident.
