I don't think hornets eat pollen and they don't make honey.They eat insects and spiders. I wonder if it was cool out and perhaps the sun was warming the steel slightly and they were on the steel or warmth?Or could it be that they are on the poles because the poles standing out in the open breeze may pick up pollen, etc that then clings to them attracking the hornets???
But I'm sure the Rangers that man the towers have seen it before, and may know the answer.