Then after that reaction, he lifts his hand to scratch a sudden itch on the back/top of his head. 

Forensically speaking, wincing, looking down/away, and scratching a sudden itch are ALL signs of strong dismay, anxiety in people. He developed that strong itch immediately after looking down/away from the evidence on the screen and trying to look casual after wincing  - a sudden itch is a common  involuntary response to suppressing strong humiliation/emotion. People often get a sudden strong itch when reacting to something embarrassing/uncomfortable they are thinking or hearing. 

Even though the prosecutor is not on trial, if he were being interviewed as a suspect by trained forensic examiners, all of the above reactions would register as "indication of strong anxiety, displeasure, concealment of true emotions". IOW, he reacted that way to the Rosenbaum evidence/video of him rioting, because it was of deep concern/humiliating to the failure of the prosecutor's case. 

The prosecutor KNOWS that his attempt to rehabilitate the rioter Rosenbaum in the eyes of the jury was likely a complete failure. That was why he reacted so strongly (with wincing, twitching and a strong itch on the top of his head) when the defense began focusing on Rosenbaum with the riot video. 

The prosecutor sort of already knows what probably killed his case - trying to make rioters like Rosenbaum et al look like normal, non-violent people being victimized. He must have known that was impossible before his case started, and having to sit there and watch the defense review what will later likely be viewed as one of the most-disastrous aspects of his case caused him to react with strong emotion that could not be concealed from observation.