I didn't think Tom quite got around to making the statement about the bottom 2 that we have used in the past to gauge whether it was the actual bottom 2 before Dorothy was called out to make her announcement. They seemed a little rushed for time. Also, I couldn't quite tell whether Tom was reading off the prompter and was surprised as well, or whether he was "in" on what was going to happen, went along with it, played it well, and pulled the whole thing off. One thing of which I feel relatively confident is that the producers themselves staged the timing of Dorothy's exit announcement for the drama effect.(Notice her placement of decision in the line-up - right at the end)
Both she and they knew she was dropping out, and I feel sure they asked her to wait until that particular moment to make the announcement.
I also feel like in return, in consideration for feelings, or for whatever reason, they went ahead and told her she was "safe" anyway. That way the whole thing went more smoothly.