I don't believe that this is a glitch in any way. This happened on Snorri. Snorri does 'Body Slam' which wipes the aggro table. It's quite possible that the high level tank had their aggro table wiped by Body Slam, and then simply by chance 'missed' the next taunt or two. Snorri is difficult to regain aggro on once you've had your aggro table wiped. If anything, this was more luck than anything else.
As someone who was there and actually witnessed it happen, i can tell you it wasnt by luck. He managed to reset snorri twice and gained aggro over our two main tanks. His low level gave him the aggro advantage and pretty much made end game taunt gear useless. It may not be a glitch, but definitely a flaw in the design of the game. A level 100 shouldnt have any relevance to the game play whatsoever on a 180 boss.