if you have a decent quality of sneak perk (50 and up would work just fine) along with muffling enhancement shoes ... the percentage of probability of achieving a kill move increases.
That may affect the probability for some, but it's certainly not the only thing. My warrior - with no Sneak at all - gets kill moves at the end of practically every fight. It's rare for a kill move to not happen when fighting a group of three enemies.
Personally, I wouldn't want the killcam to kick in every single time. It would drive me crazy when fighting multiple opponents. Having one happen at the end of nearly every fight can get tedious as it is.