I would assuming that the idea of attacking the enemy and flanking them with your forces while holding some more men in reserve to be an extremely common strategy seen throughout history. This tactic was observed throughout the world yet met with much less success, primarily due to the fact that flanking the enemy was an expected move. Why was this strategy so effective when used by the Zulus?
Well, for several reasons. One of many is that warfare on that part of the continent wasnt like your usual pitched battle involving a lot of melee.
Fighting was more ritualistic and involved intimidation through numbers and shows of resolve.
Battles would usually have two groups of warriors meeting, banging their shields and chanting war cries in order to sell their size to the enemy. Spears would be thrown and in rarer cases a charge would happen. After which the side which had more resolve and/or numbers to overwhelm the enemy would shatter the enemy morale.
Warfare, esentially, amounted to small time skirmishes and raiding, after which cattle would be stolen and in the most severe cases, a tribe would realocate to new pastures(people had a semi-nomadic lifestyle so this meant expansion of grazing grounds for the winner).
What Shaka did, was change all of that. His first, and major change wasmt the introduction of the bull's horns. That tactic was just him perfecting his introduction of heavy close-quaters melee combat and of the killing of routing enemies. His warriors didnt only have the usual throwing spear, but also equipped shorter stabbing and blunting tools and they would forego the usual meeting ritual and start the charge at the first sign of the enemy. Not only that, but Shaka also incorporated defeated tribes and tried to centralize rule, instead of the usual policy of only seizing possessions.
This new type of governance and fighting for the region had extremely major impacts, not only as it created the first proper "empire" in the area, but also introduced the possibility of higher troop organization, which lead to the Bull's horn, as commanders and units loyal to Shaka would coordinate large number of warriors through signals and couriers(a feat which requires centralized command structures when you have no long-range communications).
But this meant that this change would be adopted by everyone defeated and dispersed by the Zulus, and this lead to what is referred as Mfecane, translated as "The Crushing" or "The Culling", as this new type of warfare spread like wildfire through the region and lead to a massive spread of brutal warfare which destabilized the region.
So basically, it wasnt the Bull's Horns per se that catapulted the Zulus into an empire, but the changes in leadership and warfare that made such a strategy possible, through the consolidation of state structures and expansionist policies.
https://web.archive.org/web/20190421164229/https://www.sahistory.org.za/article/1800s
This is an archive from a SA goverment history websute
Also I used different citations from The Cambridge History of South Africa, Vol. 1. published in 2009.