It is not as difficult as it might seem. One of the potential scenarios is when hunting under a ledge.
You are laying on the bottom with your head under the hole, you are trying to maneuver the gun around and shoot a fish in the hole, but there is little room. Since you have a much longer gun than would be optimal for this situation, you have the gun mostly sticking out of the hole and you have to reach way back to the trigger and might even try to fire with your thumb.
You have your head and shoulders wedged in the hole, the surge is pushing you back and forth, you are holding your breath, the visibility is crap because the surge going around your body and the nervous fish movements, all make the visibility terrible. You have the gun laying underneath you and the shaft is close to your throat. If the fish moves and you have to make a snap shot or the surge throws you, or an eel pops out into your face unexpectedly in bad visibility and you instinctively move the gun a little further back... you can end up pointing the gun at your chin or face.
Perhaps he set the gun down to try to catch a lobster and then the lobster shot out of the hole, he chased it around and ended up in front of his loaded gun that was originally placed in a safe position, and now it gets bumped or kicked and some piece of reef is inside the trigger guard and the gun fires.
There are other potential scenarios and there are many ways that a gun, if carelessly handled could end up pointing at your face. Spearguns are dangerous and they CAN fire all by themselves if a shaft is rusty or cracked or a mechansim fails, or the mechanism is damaged or clogged with sand when loaded and only partially loaded.
