I worked at West Driefontein gold mine in 97/98. From experiance the fire could have been started deliberately. But also, it could have been caused by a seismic event. When large areas of old mine workings collapse this results in a mini earth tremor, like wise with a sudden collapse of old mine workings (we are talking large areas) the sudden pressure on the old timber supports cause the timber to heat up. The old timber can heat up enough in a few milli seconds to spontaniously combust. The old workings will no doubt be dry as the nearby mines will be keeping the water table with their pumps.