Pliny 8.18:

ideo multiformes ibi animalium partus, varie feminis cuiusque generis mares aut vi aut voluptate miscente: unde etiam vulgare graeciae dictum semper aliquid novi africam adferre

So there are born there [in Africa] diverse sorts of animals, since the males and females of various species mate either by force or for pleasure: whence even comes the common saying of the Greeks that Africa is always bearing forth something new

Pliny's talking about animals, specifically lions, leopards, and other big cats. He seems to be under the impression that the various species of big cats are produced when members of different species interbreed. I'm pretty sure he's quoting Aristotle (whom he refers to a few sentences later), but I don't actually own a copy of the Historia Animalium (it's not exactly an important or good work, and it's even hard to find in Greek online) to check

I’m not too knowledgeable about Pliny’s life but I can help you out with a short answer. This citation is from the Naturalis Historia or Natural History, which is best described as a proto-encyclopedia of some sorts. According to Pliny himself the book concerns ‘life, or the world of nature’, and the book's scope is very broad: it also touches things like mathematics, ethnography and the arts. The quote you have here is not describing political or military events in the province of Africa but the flora and fauna of the region. Romans believed that many mythical animals lived there, creatures like chimeras and such.