agen_kolar
Super Freak
Not everything plays out like it does in the books, but besides that Shae is different in the show than in the books, so her motives will be different. For me, in the show I think it may be a combination of threats, promises and the overarching hurt that she felt when Tyrion sent her away. She wants to hurt him, but other things are at play here. And who knows, they could have told her that they won't execute him,only send him to the Wall or something. She seems pretty upset and unsure when he demands trial by combat.
Yeah, I thought perhaps the Lannisters might convince Shae they'd spare Sansa if she testified that Tyrion acted alone. I was surprised that she threw Sansa under the wagon, too. But who knows what was said and done to Shae behind closed doors. She did seem unsure of herself pretty much throughout her testimony.