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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.
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.