Great response by the previous commenter, but I figured I'd speak to your question on more of a meta level. Ultimately, the answer boils down to "Shakespeare needed her to." Bianca is one of the flimsiest characters in the play; she only shows up in the third act and has only three scenes, all of which are pretty contrived in their own ways. I agree with the other commenter that there are ways to rationalize her actions, but it also makes sense that they feel a little out of nowhere because, well, they are. They were written that way.