Little Finger (Petyr Baelish) is in cahoots with Ooenna Tyrell and assassinated King Joffrey. In return he gets Sansa. That is what I think. I also believe something fishy is going on with Shae. Cersei or however you spell that hot ass wicked bitches name looked somewhat surprised when Shae walked in to testify, which would lead me to believe that Tywin in the one that found her. The only way Tywin finds her is through the eunuch I think. If the eunuch has his eyes on the crown, he has to be setting something up?!?! Maybe double crossing Tywin and aiding Tyrian? Remember the eunuch knew how to find "the mountain" so it must have been him that "got him there so quickly" for Cersei. The eunuch also confided in Prince Oberyn that he had his eye on the throne, if he knew Oberyn would exact his revenge on the Mountain thus setting Tyrian free to kill Tywin, Shae and Cersei?!?!? I don't know, it's possible on this show. Great show.
Hope this helps,
FDC