I really don't get it, its seems very random for me. I have few questions about this:

1) Is it trigger every time when the requirement was fulfilled ( which means 100% ) ?

2) Is it ONLY trigger when opponent HP drop to certain percentage?

3) Is it depend on our weapon?

4) Is it depend on the part we hit?

5) Is it depend on our position( such as sneak behind ) ?

6) Any other circumstances?

Currently I always use Kahvozein's Fang to attack a dragon when its HP is below 10%, most of the time it trigger the finishing blow cinematic when it is dying.

There does not seem to be a specific trigger. I have seen deathblows:

When I was fighting more than one NPC,
When the enemy was at full health,
With any sort of weapon (except a bow?),
Sneaking and not sneaking, and
against two enemies in a row (wolves).

So I'm not sure that there's any rhyme or reason to it. I think it's just there to signify that your character took advantage of an enemy mistake, and got a lucky killshot.

Almost every time I sneak up behind someone, I get the lash throat cutscene. Could be that my sneaking skill is 100 and I have all perks for it. Plus I always use the Blade Of Woe for sneak kills.

But I found this for the console command:

set decapitationchance to <0-100> Sets chance for decapitation. Value ranges between 0-100.

set killmoverandom to <0-100> Sets chance for finishing moves. Value ranges between 0-100.

So... If you want every kill to be a finishing move, try the console command set killmoverandom 100 and I bet every kill will be a finishing move.