Why is that monster hitting me instead of the Tank?

Everyone knows a tanks job is to hold aggro and take damage, however, something I've ran into on occasion is a player causing a huge amount of threat or on a multiple mob, attacking a monster that no one else is hitting... The problem is threat generation. On a single Boss its not much problem to hold threat and if someone takes aggro simply using a taunt to gain aggro back. Usually the problem comes with the multiple mob. Simply put if your threat goes past what the tank is dealing then you are going to get hit. Understanding threat is crucial. Here a link explaining threat.

http://www.freewarcraftguides.com/warrior_guide_threat.php

Also this addon is great for watching out for ole number one while not stepping into ole number two.

http://wow.curse.com/downloads/wow-addons/details/omen-threat-meter.aspx

Ya this holding aggro can be a bitch but as i remember tanking with my dk i just bitch about letting me have 3 sec to pull will my aoe and how they need to hit what i hit NOTHING els