Why don't you post a pic or something...that would help out if it were really something wrong...but if the duck is eating the same, swimming the same, quacking the same, and besides the occasional head to the side or twitch I would say it is probably just fine
My pet duck appears to have tourettes? does it really PLEASE HELP ME!! - Page 2
Here's the hormonal behavior as mentioned. It is sort of a bowing-down in submission. I took this video about 2 weeks ago when I noticed the Pekins starting to do it.
Pekins do tend toward agressivness. I have found Rouens and runners and muscovy's make good calm pets. Muscovy are also good because they don't quack like other ducks.