See, as a tank, we can't generate aggro on things we can't hit. And when we don't generate aggro mobs get loose. And when mobs get loose squishies die. And when squishies die parties wipe. And when parties wipe... well, things get ugly 8-(
As a tank in AoC, positioning the mobs is quite tricky because of collision detection... basically, there may be mobs stacked behind other mobs that s/he can't hit because they can't all stand on top of each other to hit him/her. Once the initial pull has happened you might see your tank doing one of two things.
Firstly, if the mobs to be tanked are all one handers, your tank will probably do a skip back or to the side to get them to re-align so they can all be hit. However, if one of the mobs has a pole-arm this doesn't work because the mob will sit out of range of the tank but still attack. In this case, the tank needs to knockback and step forward to re-align the mobs so that s/he can hit them all.
OK, to recap... the tank needs to hit all the mobs or things get ugly... the tank has skipped or knocked back and spent a couple of seconds getting all the mobs aligned so that things don't get ugly. Purrfect... game on... lets DPS.
OK, you melee dps guys behind the mobs...
...you are behind the mobs, right? Where they can't block or parry? Where your dps is highest? Yes, of course you are, sorry I even asked...
... if you knock back, you are actually knocking forward. Firstly, you are pushing the mobs behind your tank (so your tank can't block or parry) and towards your squishies (oops). Secondly, because your tank is now taking more damage (because s/he can't block or parry) the healer has to up their healing which... surprise, surprise... increases their aggro generation at the same time the tank aggro has reduced (because they can't hit the mob). Hmmm, wonder what happens then?
Now is the time you really need that knock back to save your squishy healer from the mob that's crawling all over their face, but... oops... it's still on cooldown because you've just used your knockback.
Please, all my dps buddies, all you guys who help me haul my lame ass heal and tank toons through instances... please don't use knockback on tanked mobs. Knockback that unexpected add? Please be my guest. Knockback that mob that's just broken away from me for the squishy? I will kiss your feet in praise. Knockback the mobs I'm trying to position and tank? I will throw a hissy fit and call you unimaginable names in my head even though I may be very polite to you in public... but I won't party with you again.
Just to avoid any misunderstandings, I've instanced with both my Guardian and my Bear Shaman, so I've seen this from the tank, the healer and the melee dps point of view. This isn't a tank QQ 8-)