Others have thought it might be critical hit induced - (its possible the critical hit just made it so that the above condition was met)
It doesnt have to be critical strike but i dont know whats the exact formula. I think the damage must exceed current hp + 10 ie the resulting hp to be -11 and less. Also the instagib is done via devastating attack or EffectDeath(TRUE); Afaik vorpal on the other hand leaves corspe intact.
I'm wary of gibs. Doesn't it prevent OnDeath from running?
no, edit: something like this might be written in nwn lexicon and it might be true in some older patches, but now it works and online version of lexicon was edited by me to clarify this