A setback bullet is an interesting idea. That would require less odd stuff to have happened in the reloading process. However, the recoil impulse felt heavier than a typical 134k power factor load. My gut is that it was a very heavy charge.
How many reloading for that case? I have absolutely no clue. It's range pickup brass that's been used an unknown number of times. I usually have a stock of about 15000 cases that I cycle through, and brass at matches isn't picked up. So usually my brass doesn't get used a ton of times.
I've had lots of split cases, and they have never caused a problem. I just discard them. Split cases are easy to identify because the case mouth is typically smaller than the bullet diameter, so the bullet feed step fails.