Mob Agro
If it’s anything like GW1 they go after the squishiest targets first.
I believe it is done differently in GW2 but I cannot give any specifics. It’s a bit of a puzzle.
One minute I’m completely ignored and the next I’m the only thing the Troll wants to hit. I do not believe it’s as simple as damage inflicted but a combination of factors. I suspect inflicting conditions has some role in it as well.
Still working on figuring it out.