What selective pressure exists in nature to result in a virus like this naturally evolving?

Selective evolutionary pressure is the mechanism. Mutants like this would be benign in a host (bats) and lethal in another species.

Do terrorist cells count?

Viruses do not want to be 100% lethal or have rapid action. That is bad for their spread and longevity. If a virus always kills its host and does it quickly, how does the virus spread? A virus does better when it can incubate awhile, hopefully asymptomatically, while its host goes about normal business, while the viral load increases and the contagion can spread in the open.

By the time that symptoms are noticeable and debilitating, it's game over, because the host is going to notice, and curtail his activity, thus curtailing spread of the virus and limiting its longevity.

People were so afraid of Ebola, but its lethality was a key factor in limiting its spread and keeping it rather contained, geographically speaking. The main fear about COVID-19 was asymptomatic infection and the resultant spread from people who had no idea they were carriers, and that's why socal distancing and face mask mandates were so important.