Its likely neurological. I have seen this in Silkies when they get a hard swift peck to the head. I have a juvenile that has a pretty bad bobble, on occasion when he takes a drink of water and lifts his head up, his head falls back and lays on his back. He will stay like that for quite a few seconds and then he is able to lift it back up. He wasn't always like that, I think an older hen of another breed jumped the fence once and got him good. I do my best to keep my Silkies separate from the other breeds, just incase things like this could happen. Im hoping next spring to have a fully enclosed Silkie pen.
I plan on keeping mine in a separate pen with my two hens that have leg problems because I heard they can sometimes have a hard time with other chickens. I managed to get a really good video of what he is doing..... https://www.youtube.com/watch?v=nb2npHkAKtw I would really like to know if it is something I should be worried about!! :(