I think trolls retreat when pathfinding can't find a way for them to get you, I regularly experience it when hunting them in the wild and the trees are too dense for them to get through. Feels weird to have to chase them to finish them off, but I find once I am close enough and damage them again to renew aggro they come to me. But if I don't renew aggro they just walk away. I assumed it was an AI tweek so they don't just stand still as a target, so I can see it working the same on a raid if you're in a safe sniping spot.

And back to original topic, this does include seeing them wander off into the water, but I've never seen one drown.