That’s strange… I’ve found most of the other world bosses (probably all, at this point) and none of the others are passive… if I get close to them, they instantly aggro. And I fought all of them (minus the spiders, the Rhino and these tree looking ent guys.)
Take the big spiders, for example… if I get close to their area of influence (IDK distances, but I’d say about 20 to 30 paces?) if they aren’t seeing you, they start moving around like they were looking for you and if by doing that they finally see you, they start chasing you and fighting.
The white leopards aggro from a longer distance even (or so it seemed to me…) And Rhino and Elephant ones too.
The Croc boss in the lake aggroed when I got closer but not if I stood some paces away from him, while the normal ones charged me from farther away than him.
It seems each of them has an aggro distance that’s different from the others.
Perhaps these type of guys (I saw other of them in the northern lands too… much in the same circumstances too, when I was fully loaded and didn’t want to fight) are pacific for some reason… and that’s what I was trying to confirm with my post here 
Is he pacific for everyone? Or perhaps for these the aggro distance is so low that you have to hit them for them to turn aggresive? Is the other one I saw in the norther-west forrested area of the northern land also pacific too? (I’d have to go there again and check…)
In the case of this one in the screenshot I stood almost standing in his face and pushed into him, but didn’t attack him (I know attacking him would have started a fight and I didn’t want that at that time… I was just curious of the aggro mechanics and why he wasn’t aggroing me.)