What exactly causes a finishing move to happen?

I ask because at first I was under the impression that they only occured if the final hit on an opponent is also a critical hit (that would make sense). However I have them happening when the opponent is still at full health, heck sometimes it's even the lair boss and BOOM, one hit dead. There have also been a couple times when it happened and I didn't even attack. I'd come up behind someone whose engaging my companion and all the sudden it happens before I even swing my sword.

It's almost like there's some formula that says if I am within attack range of an enemy there is a 10% chance it will go off, whether I attack or not.

It is getting really annoying and I am really close to adding a mod to stop them altogether, but I was wondering if anyone has figured out yet what triggers them.

Thanks.

I've experienced much the same, but I haven't triggered a finisher without attacking. THAT certainly seems out of the ordinary.