Those one shot kills are either from shotguns or shots in the head. I've yet to experience a cheap death from bullets in this game
Npc's should NEVER get headshots.. it is an unfun mechanic. Just imagine how many games would suck if you suddenly lost %70 of your health because the computer "got a lucky crit" on a normal attack?
It may have been a shot gun, but the game doesn't really feature a kill cam or any way of determining the weapon used when you die so it is hard to tell.