I don't know, I think she may have come to genuinely believe the hype that Corbyn was a disaster and she'd win a landslide. It was weird how she didn't seem to speak to the rest of the cabinet about the decision though, she just went with her inner circle on it.
I think this 'transiational period' absolutely is an attempt to pass the poisoned chalice to Labour though. With fixed terms, a two year transitional period dumps the explosion firmly onto Labours lap. And just far enough in that the Tories will get away with blaming Labour for not having planned or prepared for empty supermarket shelves and banned flights and tailbacks from Dover to Leeds.
Though I'm not sure why they keep talking about the transition period being a done deal, fairly sure they have to convince 27 EU nations to agree to it first, and many of them have said no way to a transition period.