Jen, I am soooooo glad that you posted this video. My muscovy ducks do the same thing, ALL THE TIME, and I never knew what was going on. What could this be?
I had a hard time watching the video (it kept freezing on me), but it looks like your females were just excited to see each other and were saying "good morning".
the head banging thing...yep that's Muscovies for you. Just their way of communicating with the others. My Peking duck does something similar but she puts her head down and moves it from side to side.
Mine do that when excited or when they feel like giving attitude to another duck. Especially the females! Lol. Notice how the one duck on the far right has her wings open slightly, as if to make herself look bigger? I always took that as a dominance thing.