My guess would be lack of oxygen. Chicken anatomy is not like humans. Their lungs are close to their back. Put them on their back and all their other organs are pressing on the lungs, not allowing for normal breathing.
I'm not 100 % on this but I believe it's not comfortable for a chickens respiratory system to be upside down? Could be a load of bull but I don't do it to mine because I don't think they like it anyway...only time I flip 'em over is if I have a randy young roo
Many parrots like to lay on their backs and relax while being held by their owners, they seem very comfortable doing it. I've even known some that liked to lay on their backs on the bottom of their cages.
I don't know if chickens enjoy it the same way, but I'd be careful doing this very much with young chicks especially, because of the location of the lungs; you don't want to accidentally deprive them of air for too long.
Ours sleep in the wierdest positions sometimes....
Caught 'em sunbathing in the run yesterday all in a big 'ol heap, like they were itty bitty chicks (they're 10wks and nothing like little any longer!). A few were on their sides - nearly on their backs - with their HEADS UPSIDE DOWN!!! Yep, completely turned wrong - way up. Wierdest looking thing.
Of course, our dog had to 'awaken' them because they weren't clucking around like 'normal'.
So, yes, ours sort of sleep on their backs - well, at least their heads are!
I doubt its from lack of oxygen, location of lungs not withstanding. Birds lungs work very, very differently than mammals and are quite a bit more efficient. No diaphragm. They have air sacs that are filled as they breath and pass air through the lungs both as they inhale and exhale. If you restrict their chest so they can't inflate their air sacs you will have a problem and the can suffocate.
I always thought that the quieting down you see when laying a hen on her back was a self-preservation response to being caught by a predator. Laying a chicken on its back is a very abnormal position. One you might be in if a predator were to grab you and pin you. If an animal struggles when caught the predator will only become more aggressive about the kill: clamp harder, shake the victim etc. If the animal lays quietly the predator may think that the animal is dead and may relax or let go of their grip thus allowing the prey item to escape and see another day.
I found my chicken laying on his back this morning. Freaked out thinking it was dead lol. Picked her up and gave her a cuddle and put her on the ground. She was definitely a bit disorientated as she fell over a cpl of times but then just walked off like normal. Hopefully she's fine maybe just sleeping like u say but a little bit worrying lol.
Good question and great answers. Many truths in these words. My truth is this: I am going to be doing a bit of bumblefoot surgery (ashamed to confess this). The local cat/dog vets will not sell me even the simplest of surgical tools, like a double-ended House curette or a biopsy punch. The farm supply stores are so limited in what they sell, really not worth the mention. That said, anesthetics are out and anesthesia (sp?) is not recommended even if I could even afford it. And so, I am grateful the Lord made birds to be calm and relaxed when put on their backs (and their sides). I'm fine with that and hope it stays that way.