The reason is an agreement in principle that she will replace Kris Faafoi as Chief Press Secretary to Phil Goff.
However the timing of this is very is interesting. You see Labour have yet to have their “democratic” selection process. Yet the outcome seems certain enough that Fran has quit prior to the 18th of September when the selection is made. I guess, the head office delegates are not going to be listening to who makes the best speeches on the night.