I wonder if there is any one particular thing that triggers the fighting animations when you kill someone, or is it just random? I love 'em! Nothing like seeing your character running a sword through a bad guy...or gal.

Without mods, it is usually a power attack on the last enemy. You have to be in a certain range though for it to work.

http://www.uesp.net/wiki/Skyrim:Combat#Finishing_Moves

This may help explain it.