Maybe I am a bit slow today, but I don't understand how this is grounds for a mistrial.

If I am understanding what you are saying is that if a defendant has a reaction/emotion to a witness during a trial then they can automatically receive a mistrial? Or is it that the prosections is asking witnesses how the defendant acted during the time in which the crime was committed? If it is the latter I still don't think they will get a mistrial as the witnesses testified as to what Casey was doing, where she was going, what she was saying about Caylee etc. It was testimony as to her actions and lies which are all a part of the alleged cover up.