I think we all have the same issue there and it's probably the complex bot code pumped by EMU that handles that. I don't know for sure, but it seems like I read that ... the code is so touchy that server admins seldom touch it. /shrug
My live player is a Pally and I use a War who hold aggro fairly well, but not completely. Two prevailing factors are Hate Rollover and BOT versus live player aggro. You can only build hate for so long until it "rolls over" to zero and you start building again. The bot versus live thing... If I stay out of the fight to start and then enter after a few seconds I always take aggro from the War.
I also have a live War and his pally bot never takes aggro, but the Rogue and Monk do all the time. I also have a live Mage and she never gains aggro until the bot War, Monk, Rog and her pet are dead.