I've seen a few videos about chickens being laid on their backs for a few seconds after which the chickens stay in that position until they are turned over. Comments around the video suggested that lying on their backs is a very unnatural position for most birds and so they don't have any reflexes or instincts that would help them get back up. But this is only speculation. Why is that really happening?
Here's one video demonstrating the effect: https://youtu.be/R9mzz9jxNsM
I've been told Chicken hypnotism is related to this.