There is almost never a single cause, here there was 12, it is often called the swiss cheese model. The root cause is a bad transcription, which probably happened many times, but for some reason, this time, all the safeguards failed. It happens sometimes, with catastrophic results. Hopefully, procedures will be adjusted, but in general, you can only minimize risks, not prevent catastrophic events entirely.

It was an expensive mistake, but thankfully, no one died.