So that guns don’t just “go off” in your pocket. Of course, the gun didn’t just “go off”; in fact I’d be willing to bet dollars to pesos that when the person in question retrieved her hung coat from the tree limb that their finger or some fabric got bunched into the trigger guard, resulting in the negligent discharge. If you carry a pocket gun, get a pocket holster for it.
With pocket guns, the purpose of the holster changes from weapons retention and transportation to primarily being there to prevent NDs. The trigger guard should be covered by a proper pocket holster, and most importantly don’t put anything else in the gun pocket. If there’s a gun in your pocket, it (and its holster) should be the only occupants of that pocket.