#wtf......

No clue, I am afraid. That has to be one of the creepiest things I've seen so far.

I'm pretty sure it's tied to the scene where the Witness is shown going through the beam and makes some sort of a hand motion:

Which then immediately cuts to the scene of the slicening:

Note that in the footage, you can also see the Guardian getting sliced. Hard to see in the image, but in the footage, if you look at the helmet, you can see it also being sliced apart. Also the Ghost is fine at the start and then suddenly shuts down and THEN gets sliced in a really terrifying display of slow and methodical motion which adds to the creepy factor: