A general rule in canids is that bigger canids kill and persecute smaller ones. In this case, black-backed jackals are killing off Cape foxes.
That would not be such a problem, because leopards keep jackal numbers in check.
This is a great example of the mesopredator release hypothesis, which holds that when larger predators are removed from an ecosystem, the smaller predators become more numerous and the species they normally kill suffer as a result of this imbalance.
Of course, the ranchers don’t want jackals either, but I guarantee you that they’d rather have jackals than the leopards.
When top level predators are taken out, all sorts of things happen to the lower trophic levels.
Increased predation by jackals on these two species of fox is just a consequence of removing leopards from the ecosystem.