Although the dominance angle sounds cool, I think they are just stretching.
This morning, I watched a single gobbler stretch and flap after feeding under my birdfeeder. There were no other turkeys around to dominate or exert body language to.
My theory is that after feeding for a period, they get what amounts to a kink in their neck or between the shoulder blades and just need to stretch. Ditto for just coming off the roost after having a pair of wings folded on your back all night. There may be something to the yawn factor that Ozark mentioned, something akin to "that's a good idea, I need to stretch too."
"The joy of living is his, who has the heart to demand it." Teddy Roosevelt