Monsters attack party members when I have aggro

[Rymm]

If you are aware enough to notice this happening and conscious enough to be concerned about it, it sounds like you are doing fine with tanking. That's like 90% of the battle right there with most new tanks. At this point, you should try to familiarize yourself with boss and mob mechanics. Good tanks know their job inside and out. Great tanks know the specific encounters inside and out and how to play to their maximum potential in any given fight.

Except tanks. It cannot be called fully "random" if it excludes the person that is top of the list.

Well that depends on the specific boss fight. Niwashi's example of the Aiatar (final boss of Brayflox) truly is random, as he will occasionally spit poison at the tank. Hellbender's bubble though, is anyone but the tank. Then you have Suza Ex whose target-a-different-player attacks obviously go after certain classes, e.g.- the purple circles in phase one always target the 4 dps. Any given fight can have any or all of these type of mechanics.