All of us here at FR know the reason she had to go under the Bus! BO couldn’t have her being asked any more questions, especially in a Senate Confirmation Hearing!
First She goes from net to net hawking the video lie (one suspects at the WH's direction)
Then, (again, one suspects) in order to keep the WH insulated as originators of the lie, Rice obediently throws herself into the (metaphorical) wood chipper to keep from having to face difficult questions during confirmation.
If true, she is the very definition of a good sport.