I recently visited a power plant where the sulfuric acid, and maybe the caustic soda, too, was moved by vacuum from storage tanks to the demineralizer beds for regeneration of the resin. The engineer giving me the tour said his company didn't like pumping those liquids and had much better luck...
I don't think I have any sure-fire ideas about what is causing the oxygen depletion, but perhaps you could do a field experiment using some of what has been already suggested. Maybe, based on location of the site, security, instrument availability and so-on, the suggestions are impractical, but...
I forgot one other possiblity. Perhaps the victim collapsed not because of the atmosphere in the manhole, but because he was ill. He may have had undiagnosed heart trouble, or could have suffered a stroke.
A tragic accident, and I want to express my sincere sympathy to the victim's family, his friends and his co-workers.
I'd say good training was the reason the other men didn't try to go into the manhole and rescue the first man.
My thoughts below are probably only some that have already...
I've seen what seems to be denickelification of Monel alloy tubes in high pressure feedwater heaters in an electricity generating station. I can't tell you much more except that it is a known phenomenon even in this high-purity steam/water environment.
