The April 9 emergency landing of Eagle Air/Air New Zealand Beech 1900D en route from Auckland to Whangarei, attributed to an open rear cargo door, has been blamed on untrained baggage handlers who did not lock cargo doors.
The hatch flew open on takeoff. See the link to a photograph of the open door posted on the NZ Herald
The doors tend to be left unlocked in case late bags come in; but that is not a good safety practice because a broken cargo door can smash the tail of the plane or worse.