I've noticed the more often you use a type of weapon the more often killcams will appear. Meaning, when i boot my console up and jump in to play it might take an hour before I see my first killcam with my fists/warhammer, then they steadily increase in frequency for the next hour until about every 3rd enemy triggers one. Will stay around that frequency until I stop playing...which during my days off is usually 8 hours+ a night.