Fixate mechanic. What I *think* happens is that they fixate on the first person to generate threat after they spawn, so it's always the healer if you're using the shuttle, but stacked on the boss the tank has a chance with a well-timed surge/sweep/slow time to be that first person. Taunt will override it, but only for the duration of the taunt, once the taunt is up they're going right back on the fixate target.
So yeah, the healer was a tard and wasn't a damn thing you could do for another 39 seconds.