I for the most part in these past few years have been playing non tanks. (Have a char that was able to act as a semitank but not anymore, still thinking how to make that available to him:)
.)
My ranger uses mainly the bow.
He has not yet been to Sithicus, and have only some exp with him regarding Harakir, Ghastria, Vestibule and Mists.
What I have noticed that he gets aggor usually if he is the first to shoot his target. Second if for some reason the enemy gets flat-footed or deaggrod it is very easy for him to get aggro. (rapid shot)
Thirdly natural 20 will get you an aggro if it is a successfull crit. Because of the x3 multiplier.
The way to counter or mitigate this is usually to restrict the opponent in some way, or be able to hold your own until rescue can come.
Called shot because of the movement restriction is quite useful in this regard.
This aggroing does not just apply for ranged chars though.
My cleric is a healbot, if he uses DPS/Buff spells or special feat/talent that the opponent can see and the tank has not yet engaged blows with him, he will get targetted.
Turn undead, warding gesture, flamestrike, hammer of the gods, BUT EVEN BATTLETIDE works this way. Anything that targets the opponent also, i am not yet 100% sure if mass party enhancing or healing is in the same regard, as sometimes using those also trigger an aggro shift to my char.