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Given that the diver was stating that he felt exhausted, this would be a good guess. Not that age is a disease, but at 74 years old, exhaustion should have been enough for him to reconsider diving that day. Also, the OP seems to think that there was a guide with the divers, but there are many dives ops that don’t put a guide in the water. Since none of us know which op he was with, we’ll probably never know. Not sure what the norm is in Maui.
Not the oddest thing about this case:
This is why many of us ascribe to the philosophy that "Every dive is a solo dive".
By far the most common reason for a scuba fatality is a medical issue, like a heart attack. Heart attacks are also the most common cause of death in bowling and golf. Have you heard of bowling alleys or golf courses going out of operation because a customer had a heart attack?
Dive agencies train and certify dive professionals (divemasters and instructors). They do not control the conditions of their employment, though. A local dive operation typically employs divemasters and instructors, and they tell them when to work, where to work, and how to work. Some employers demand that their employees act in a highly professional manner; some do not have such high demands, and some will even tell their employees to violate standards to save a buck.
There is probably a reason DAN won't insure people of that age.
Actually, they do.
