How might disease be responsible for the rise in buffalo population that our investigation is centered on

Suppose that the buffalo has a natural predator A.

Now, if there is new disease introduced, that particularlly atacks the natural predator A of the buffalo, we may be in a situtation where the percentage of the buffalo population that was predated by A is decreased (as we have a smaller number of predators A that are active). This means that we have more live buffalos, and as most biological organism, the growth of population is exponential. So having more buffalos today, means that in some months we will have plenty more of them, and this is why a disease can conclude in a rise in the population of some species.