OK, i think we've been given that clip WAY out of context. My guess is that Tarkin does something to kinda screw over Ahsoka, put her and/or the Jedi in a bad light, or Ahsoka may be ticked off due to something that happened with Letta and Tarkin ain't helping. Barriss probably just feels bad for Ahsoka for any one of these things or possibly a combination. She's been set up as Ahsoka's friend and nothing else so she feels for her. Remember what they went through in Brain Invaders. Its possible also that Ahsoka is freaking out and Barriss feels bad for her but thinks she should settle the f down.