The Zulu saw Chelmsfords col coming they then left, crossed the drift and headed for home. A small time and motion study would put the crossing either on the plain leading to the drift or at the vallet as the road bends to the SE.
Another clue is the col was seen from the roof of the store room, that would put the col someway across the plain.
The Col had spent the night in fits and starts sleeping amongst the dead and started before dawn on the march back to RD. The prievious day the Red Baron had them chasing shadows the whole day, sections had been involved in skirmishes.
They probably had 40 to 50 rounds average per man. Enough to fight a fair size battle.
They didnt, as we well know fight, so boils down to a lack of moral/willpower. They were mentally and physically exhausted
The zulus had had enough hence the reason for leaving RD, they were in no mood for a fight. There ammo was probably exhausted.
Only one Zulu was keen but he couldnt raise any interest with the Amabuthu so he attacked on his own and paid the price for being pretty damned stupid.
There was also the supernatural aspect, they thought the returning col could have been the ghosts of the men killed at iSandlwana.
Hope that helps.