Did the first tank immediately die after scream finished casting?
If so, this is probably due to the damage scream does when it lands on the tank. If the tank getting screamed is not topped off, he could still die from this, which may seem like your taunt didn't work.
So to be clear, when scream is finished casting, it will land a big chunk of damage on the first tank. It then switches to the second tank that taunted and continue regular attacks. Healers should be prepared for this.
On HM, this scream usually does close to half my tanks HP in damage (aprox 11-13k). It's probably weaker in story, so your healers got to step it up. Or maybe your tank isn't moving out of the spit and taking unnecessary damage.
Hmm, thanks. I must admit I did not watch the other tank's HP at this case as there's no "emergency taunting"; I must taunt during Scream, irrelevant of how much HP he has. So it's a possible explanation, yes, as he did die immediately.