It is caused by punching through punchable surfaces or enemy
In your video, you try to gk the stagger hell knight but accidentally trigger blood punching because of another hell knight
But your momentum still pulls you to the stagger one and makes him die
Punching in this game pulls you toward the enemy (And punchable surface) and Then stops when your model hits the enemy's model
but if you punch a corpse (1-2 seconds before the corpse disappears) you will be pulling toward them but there is no collision to stop you
that's why you get yeeted to the air
I don't know if you understand what I said or not (I don't use English as my native language)
https://www.youtube.com/watch?v=kWF_-yFLhiw