Ronda, with less than 2 minutes of professional cage time, decided after the Budd fight to drop to 135 and fight Miesha Tate prior to going back up to fight Cyborg at 145. That was the plan. If that's "ducking", then so be it, but if she had instead decided to try for Cyborg's title right away, the fight would have never happened anyway because Cyborg got popped for roids, suspended, and Strikeforce then dumped the 145lb division altogether.
Since then Cyborg has been doing the ducking, by taking no affirmative steps towards securing a fight with Ronda except tweeting and complaining. She could have signed the UFC contract she was offered, or started the process of dropping to 135 in another organization, but has done neither of those things.