Attributing a chemical safety incident to 'human error' is like blaming a fall down the stairs on 'gravity'. People don't fail, systems fail. Those poor guys were on top of the road tank wagon because it was their job to be there.
They may have been ignoring procedure, but we need to ask why? HF is one of the most dangerous substances handled in the chemical industry.
The management oversight, process control, safe system of working and audit process clearly failed here.