Noel Mostert’s Frontier goes into depth here. At the time, the Xhosa had a heritage of prophesy. Turning bullets into water is a famous example. I suspect that the 1848 prophesy arrived when herds were under threat from the new disease, and territory under threat from the Zulu. The Xhosa are outnumbered by Zulu and both tribes migrated into the area in the historical period.