Try attempting to do it while on level ground and not moving. The detection to do the bite isn't the best.
But yea, they need low health and you need to power attack, as said above.
I thought it was "random" and only happened on occasions, very much like the death blow cameras.
It only seems random because it is very inconsistant to perform the bite finisher. I have had it where just about every kill was the bite in several battles, and in others I can hardly get it to work.