My guess is that Juan knows for sure that there is child porn on that computer. And, of course, how would Marc knows about the details of those revolting images unless he was the child porn viewer? If I even came across such nastiness I would immediately close those sites and tell someone. I wouldn't look at them long enough to remember details. If Travis had really watched child porn then Arias/Marc would have made sure to focus on it every single day during the first trial. I think Marc recently remembered that he had looked at child porn on the Bishop's computer and realized that he could try and blame poor Travis for it--hence the ever-changing dates and testimony. This kind of evil is nauseating.
Or else he was content to flip it and let the jury draw the conclusion that the reason this witness is afraid to testify is because HE is the child pornographer.