The guy in lead is running away from a skirmish and has low health so I give chase, but for some reason everyone else thinks I'm a better target.
Same thing for when he's capping a chalice, fighting the arena monsters, or doing chest runs. Being in lead apparently makes you the lowest priority target.
Someone running to activate the trap on Forums of Hercules and I'm running right behind him close to grapple range, yet the other 2 guys think it's better they kill me. The list goes on and on.
I've seen it all before and I use it all to my advantage. Simple answer "Tunnel Vision". Countless times i've dodged through someone to see the person trying to kill me instantly swap to him because he's 'closer'. I'd assume that the majority of players who do play with tunnel vision don't even know what the score is until the game is over.
Think about it like this. Guy has low health and you're chasing him. I see that you're about to get an easy kill (which means you get easy points), so I choose to stop you instead. Personally, I'd just knock you aside and steal the kill for myself, then come back and find you. As for not stopping the guy with the chalice, that's just players being dumb. The player with the chalice immediately becomes my #1 target.