Which character? Your Classic mage, your Shadowlands priest, or someone else?
You know what? I’m going to go out on a limb and guess this is occuring on your priest, since you went to the trouble of changing your posting character to him.
Your priest talented Surrender to Madness as his Level 50 talent choice. Let me quote the description of this talent:
If your fight against any tough opponent takes longer that 25 seconds, that talent instantly kills you with no reprieve and no second chances. In which case, working exactly as intended.
I mained Shadow Priest for a while before switching to this hunter, so I’m keenly interested in SP playstyles. StM was always an intriguing talent choice, but utterly impractical for me for day-to-day play because of that whole dying thing. This version is friendlier than previous versions, which didn’t allow you to escape death by winning in a certain amount of time. But it’s still too much for me. I hate corpse running.