Are you talking about a kind of Stevie Wonder kind of head bob sort of? LOL I don't know how else to describe it.

If so...both of my Roos do this every once in awhile, during the day and have for a couple months now since becoming mature. I assumed it to be normal Roo behavior. Kind of like how they stretch and flap their wings once in awhile every day too. They don't appear annoyed or in distress so... unless someone smarter than me on here says different...it probably belongs on the long list of weird things Roos do. Hehehehe
A chicken with something stuck in their crop, by the way, will 'yawn' and stretch their neck out, and they'll do it a LOT until they work it down.