Just in case anyone is still struggling with this issue. Pretty sure it is not a glitch but an effect of the disintegrate spell. Spell is damage over time so in a busy combat log the intitial message that a character has been hit by it can be easily lost and by the time it insta kills it can look like a character has died for no reason. The spell in particularly pernicious because if the character's health reaches 0 for pretty much any reason while affected they die.
Here are some tips for mitigating against this.
First; keep an eye out for the spell and if your characters are under its effect (it has a pretty noticable graphic when its affecting a character once you know what to look for)
Second; buff! There are three things you can buff that help: (i) health (via constitution buffs usually), (ii) spell resistance, (iii) fortitude (the disintegrate spell is vs fortitude). Constitution buffs help with i & iii so if nothing else a rest with your highest con boosting foodstuff is a good start.
Third; once a character is bloodied or below healing them should be a priority, especially if they are a low health chracter. If, like me, you tend to have only one healer in your party then stock up on potions and scrolls.
Forth; for mages you can use arcane reflection (there are also potions of this as well as a lesser version) to provide protection for some period of time.
Finally; if all else fails the priest spell (and common scroll) of withdraw can be used. It will take the character out of play for 20s but it instantly stops any damage over time effect.
