Now, we see the obvious result of what was going on, but unless we know the exact building process, we don't really know how it happened. I've seen lots of videos of engine building where they use automated torqueing devices and where a computer selects individually spec'd parts to be used in a specific place. That, in itself could open up another can of worms, but realistically, without input from the builder I don't think we have a good answer.
It might seem to be an aberration, given the number of good engines out there, but IMO, I would have this incident immediately scrutinized by the GM Performance Build Center at Bowling Green in case there are procedural things that might need attention.