Greetings,
I am not sure if this actually IS a bug or not, but since aggro mechanics are not published for this game, I thought it worth mentioning in case it happens to actually be a bug.
I play an Engineer in the game and almost always play with my wife. My wife has several alts, but mostly plays her Mesmer. As we have played, we have noticed that she never gets aggro and that I always do. Even if I have not fired a shot or used any skill, once combat is initiated, I am targetted by whatever MoBs happen to be in the area at the time.
I have posted several examples under this post: https://forum-en.gw2archive.eu/forum/professions/engineer/Are-engineers-agro-grabers/first#post492798 and noticed that several people were stating that is was not only Engineers that this was happening to. I have read posts that state that several times MoBs would go after even AFK characters instead of the ones that were actually attacking them.
I wanted to report this as a bug as it is seriously inhibiting my enjoyment of the game. I do not mind a challenge and certainly am enjoying the active combat mechanics of the game, but it is getting tiring to ALWAYS be the target of every MoB in an area, no matter what else is happening.
I am also not the only person to notice this. My wife has noticed it on many occassions and I have received tells from friends and acquaintances in the game asking me what I have done to anger the entire zone so much!! 
Since none of us players know the aggro mechanics of the game, I am putting in this post so that the mechanics may be looked at by people who do know them. Several of us have tried to figure out what is wrong, but cannot and we need help to get this issue resolved.
Thanks.
“You can get more results with a kind word and a big stick,
than you can with merely a kind word.”