just about every weapon class has new killmoves in 1.5, there's no need to be a [censored] about it.
and he's right, it's random. want more killmoves or for them to happen more frequently? download the Dance of Death modification.
Actually, there's a console command to set it to a higher percentage. Let me see if I can get it. I think it's:
set Killmovechance to x, where x is a number from 1 to 100. And even at 100 (which is 100%, it's not always going to work.
I wish there were more Killmoves for the Greatsword...
l