If the witness was hunting, I guess he was armed?
Yes, he would have been armed. But as he was hunting ducks (birdshot), he may, or may not of had ammunition capable of stopping an armed human.
Dementia very well could be the cause.
Possible, but there is also the possibility of too many hunters in too small of a lake with not enough ducks. Though I am not familiar with duck hunting (it sounds like fun), the lake evidently has an awful lot of hunters.
Then add possibilities of some not following etiquette regarding who should shoot at what ducks, who is, or is not "local", and some who make territorial claims to public property, egos etc.
Then factor in that not all rules are written down, the unwritten rules can change and some humans routinely exempt themselves from unwritten rules they expect others to follow and things can get heated.
As a side note, surfing also has the same elements and violent surfer "beat downs" happened yearly when I lived in SOCAL. The root cause to the violence was that surfing had exploded in popularity and there were too many surfers and too few waves (well, too few waves of the surfable kind).
Then add egos to surfing's, unwritten rules, territorial "claims", unstated "seniority" privelages etc. I think the only thing that prevented a shooting incident was that surfers need to get
in the water- and that is not good for firearms.