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.