Thread: Brass splitting

My guess would be it happened only after the sizing stroke and when the expander plug was pulled back through the sized neck. Either the neck's interior surface wasn't lubed or was insifficiently lubed. Mostly this would cause the plug to bind and take noticeable effort to pull completely through. It's rare for neck separation to occur in such instances so I suspect your comment about long term poor storage of the brass probably has merit.
Anyway, better to lose that substandard case now rather than when being shot.