>

Wait, really?  Why?
In order for this happen the gun had to be
1) Loaded
2) Chambered (and/or cocked depending on the type)
3) Safety Off
4) Left out in the OPEN in the BACK OF A TRUCK where a dog could step on the trigger/action while being ready to fire
5) potentially with the muzzle already pointing in the direction of the person 
This is so irresponsible it sounds almost impossible to believe. >

We had a dude in my hometown who rolled his pickup in a car accident (his fault, he ran a red light). This asshole had a shotgun just sitting loose on his backseat that went flying around the cabin when it rolled and discharged because he was a dumbass and had it loaded and chambered with the safety off. Thankfully, no one was killed or injured, but the shot hit a nearby residence with people inside of it, which made the charges against him far more severe. >


because he was a dumbass and had it loaded and chambered with the safety off

It's possible the safety was on, but this is why you're not supposed to keep a shell in the chamber of a shotgun. It doesn't require a trigger pull for it to discharge. Most pump shotguns have firing pins that can impact the shell without the trigger's involvement.