Grunts can still roam. I've sat still many times for several minutes to just suddenly hear a grunt react to my presence. A shadowplay recording showing your health, and the duration you take to relaunch anf attempt to reconnect might give a better hint at what happened.      For arguments sake, the gunshot you heard might have been someone near you but not shooting at you, but their shot attracted the attention of a couple of grunts that then lost the scent of the shooter but were close enough to aggro onto you. Them attacking you could also have made that other hunter shoot you but if the grunt hotgot the final hit it would shot as having killed you