Brutalus' Burn ability temporarily sets his current target to the player that gains the Burn aura, then resets his current target to the previous target. The duration of this target change is roughly 500ms to 1 second.
If you taunt during this time while targetting brutalus, your taunt aura is applied to the person he temporarily taunted, not the other tank (his previous target).
Somehow then the server logic ignores this, probably to prevent the ability to intercept burn debuffs by using taunt.
Anyways, what happens is, nothing. Your taunt is used, brutalus is still attacking the other tank, and you need to wait 15 seconds before taunting again.
It is difficult to control because of latency issues and how your client is on a different computer than the server, and the algorithms used to synchronize the two. You can tell when burn is about to come, but even if you taunt 1 second after burn the server may still be in the state where he is targetting the burn victim and not the other tank.
To prevent this, you need to wait probably at least two seconds before and two seconds after around the time burn is cast to prevent wasting a precious taunt.
There is probably nothing Blizzard can do to fix this short of changing the mechanics of the Righteous Defense skill entirely.
_________________
