"The worker's employers say an initial investigation shows the incident was unrelated to safety procedures, but it is not yet clear how it happened."

This seems plainly related to safety procedures: either they weren't followed, and they were able to be not followed in a way that led to death, or they had a gap in said procedures that led to "correct" actions leading to death.

The alternative is it was intentional, which while possible generally results in articles saying something like "It is unclear what happened.<paragraph break>Hotline numbers if you need to talk to someone are ..."