Don't beat yourself up, but it would be good to find the cause if possible. As Monte said, sometimes it is impossible to know for certain.

If I had to bet, after reading through all this, something in your oil system was amiss. If it were my engine I would carefully inspect all of the oil passages for places that were broke through, missing plugs, etc.

Given the nature of cast iron it's not inconceivable a thin section of drilled out passage could break out just from oil pressure.