I know how you feel. That happened with me when I reformed brand new Remington .30-40 Krag brass into 6.5x53R Dutch, after only one firing. I attributed it to the Remington brass being thinner, there wasn't that much case stretch or expansion after firing, basically the same compared to my Boxer primed original military brass.
You mentioned backed-out primers, were they flattened, or just backed out easy? Backed out primers aren't necessarily a bad thing by themselves, often they indicate too light a load. 2421fps for a 150 grain bullet out of a .280 AI seems a bit sedate for that round. This has happened to me in both 8mm Mauser and .30-30 Winchester, and I freaked out at first, thinking it was either too hot a load, or a gross headspace problem. Then I simply incremented up from that beginning load by 1/10th grain at a time, and the primers stayed in place. The Speer and Accurate Arms reloading manuals make mention of this.
If it was just that one round that split apart, with no stretch rings forming inside the remaining cases, I'd say it was just that one piece of brass.
I know when going to an Ackley shoulder, the parent case's shoulder is very important for headspacing in the larger Ackley chamber. Maybe a smidgen less "bump" on the resizing die will give a tighter chambering against the "false" shoulder before firing? It's how I fireform for my 6.5-06 using resized RWS 7x64 Brenneke brass.