I've noticed that when fighting world bosses (actually it happens a lot), sometimes the boss will focus on me and only me. It must not be based off of a first strike (I don't usually strike first unless I'm on a nightblade trying to get a stealth bonus or invisibility critical), and since I don't typically use a lot of dots or rebuffs on WBs I can't imagine it's damage related, so how does the game determine who the boss targets?
"'You have suffered for me to win this throne, and I see how you hate jungle. Let me show you the power of Talos Stormcrown, born of the North, where my breath is long winter. I breathe now, in royalty, and reshape this land which is mine. I do this for you, Red Legions, for I love you.'" The Many Headed Talos - Michael Kirkbride