At first it was researched if the virus showed signs of genetic modification, which it lacked the typical human-made code chunks for, so the hypothesis was discarded. But then a scientist realized something, in pharmacological research there exists a method to make animal experiments a more accurate model for humans by modifying the animals with human DNA. This makes rats susceptible to human diseases like HIV. That way you do not have to modify the disease to fit the animal, which could change the behaviour of the disease and thus the efficacy of the tested medication.
What the scientist discovered was that you could inverse this process, instead of making the animal more susceptible to the disease, you can force evolve a disease to become virulent to other species. For instance, by gradually hybridizing bats with human DNA you can evolve diseases to become infectious to humans.