I think what happened during the debate is that Hillary had talking points to try the crazy nuclear Trump attack line, and they probably told her that she needed to impress everyone with her experience and knowledge.
However (and this is where the staff really messed up) they did not tell her _exactly_ what to say because _they_ relied on her experience and knowledge and judgment.
Big error!
Error!
Error!
Either that or she was told to say that because no one on her staff realized it would be criminally negligent for her to announce to the entire world.