This is not "random" or "autokill".
Its simply the survivor getting a headshot.
No matter how much health the hunter have, he WILL die of a headshot.
Thats also how skeeting works. Well and the shotgun shoots 8 pellets and would still kill the hunter if they all hit the body, but for SMGs if you get a headshot the hunter will be skeeted with only 1 bullet.
Now the fun part is on how to block this
Ofc SDKHooks with OnTakeDamage is a good candidate, but I wonder if you can somehow only block headshots with it. Hm...