Lack of proper gun safety.
One thing we know for certain is that it was the lack of gun safety on the part of the person who pulled the trigger.
But on the basis of what we have been told in the story, we cannot tell anything further.
Sure, it might be as simple as an elderly gun owner who handed his weapon to someone with the magazine removed but left a round in the chamber and assumed it was unloaded, and an inexperienced person pointing it where they shouldn’t have and pulling the trigger.
But given the details, it could be anything. For all we know, he may have been teaching target shooting and the kid turns and shoots him.
I mean, look what happened to Chris Kyle. People can be strange.
It isn't hard following very simple, basic rules, and those rules WILL save lives, every time they are followed.