For them, the message/point the opponent is attempting to convey is so damaging to the Dems narrative that they must obstruct it. After such an outburst, the Dem looks "justifiably upset" to the Dem base because of whichever disadvantage card was played and the painful truth of the opponents point is lost to the audience because of panelists trying to talk over each other. Essentially, in this case, Deep State Donna comes out of the discussion in much better shape by drowning Tammy out than if she let her speak the damaging truth unhindered.