I had a weird similar thing happen with some of my reloads a few years back. It was a favorite load in the 25-06 that I had used for years. I don't recall the exact load, and it's not important anyhow, but it was with a Nosler partition bullet and I had used this load for deer for years. Suddenly I went to check the zero of my rifle one year before deer season and the load was way over pressure all of a sudden. (These were cartridges that had been loaded for several years, by the way.) The bolt handle was real hard to lift, primers leaking around the edges, ejector hole imprinted on case head and I couldn't figure out what was going on. I even put the 25-06 aside and switched to using my 270 to hunt with all that year. I finally figured out what was going on when i decided to pull some of those bullets. There was bad corrosion in the necks of the cases which acted to partially bond the bullets to the case and cause a big pressure spike. I had to really get rough with the kinetic bullet puller to get a few bullets out to discover this. I finally fixed the problem by adjusting my seating die to barely bump the bullets deeper into the case - just enough to break the corrosion free so i could shoot those loads. As to the real root cause of the problem, I don't know what was different in that batch of components that caused the corrosion in the 1st place.
