360 To perform a finishing move on command is done by tapping RT and moving the left anolog stick at the same time in any direction, which will result in different finishing moves.
From: http://elderscrolls.wikia.com/wiki/Finishing_Moves
Maybe some truth behind it, I use 360 controller on PC, and notice I get KM's more when I am moving around (moving the stick), although I have noticed that if I am quick enough (happens alot by mistake and it's annoying to see it happen, cos I love the KM's! Take THAT!!) I can cancel out of the killcam, usually happens when I press RT twice for two swings of the sword, the first swing procs the killcam, but before my brain cancels the signal to my finger to not hit RT, it's hit RT and I'm back in combat as if it never happened.
Maybe more investigation is needed in this area.
Also, I forgot to mention it above, but it appears KM's can proc alot more frequently if you find yourself dominating the combat, as in, fully dominating your enemies. Not necessarily by just uber-pwning them, but I mean by just simply outdoing them, blocking their hits, countering attacks, etc.
I use Deadly Combat so it's hard to totally control the way combat plays out, there have been times that I have been dominated by a Bandit before, simply cos his AI cycles got lucky, blocking just as I am swinging, etc. If you're on PC, definitely try the mod out, it changes combat so much. Clearly all my opinion though and I don't even know why I let myself type away like this, or why I don't just delete it all and leave the good bits, meh, hope you enjoyed reading this as much as I enjoyed typing it up, which isn't a high target of enjoyment. I'll just click Save Changes now.