That's pretty funny and bizarre! I've never had a temple monster activate a trap before, so I would be pretty surprised if that was the cause of his death (but I'm pretty surprised, anyhow). Usually they just walk on the trap, but everything's as if it had walked onto a normal square.
Unless I'm mistaken, items can be generated on the left side of the air temple level. If one of those items happened to be generated inside Yulgash's room, and it was a nasty potion, like cure corruption or sickness, that could have been the cause of his death. Monsters have the ability to drink potions that they pick up.
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You spend some time with Skriek, the cloaked ratling, who seems to enjoy your company. The sky is now very clear. The sun burns down on you.
You talk to Skriek, the cloaked ratling.
"Yer alternatives are locked, wisehead!" Skriek, the cloaked ratling, misses you. Skriek, the cloaked ratling, misses you. Skriek, the cloaked ratling punches through your armor and hits you.