Grave robbers worked at night so as not to be seen, but this also meant that they may not see these guns mounted on a grave, and would probably not see the trip wires.
Several trip wires could be fitted to the trigger mechanism, running in different directions. This not only covered a wide area, but it also meant that any one of these wires could swivel the gun in the direction of the intruder. (h/t)
They were referred to as "Grave Guns" or "Cemetery Guns".