Guild Wars 2

I’ve had the Temple of Grenth event fail multiple times even though the NPC Jonez is still at full HP. It seems he randomly calls out about being outnumbered and retreats. What causes this event failure?

Most of the time what happens is that some clueless player drags the priest over to Jonez and an icicle proceeds to drop on his head which can kill him (sometimes instantly).

I do not think the shades debuff effects NPCs, so I do not think its that.