I've seen two of my 10 week old chicks do this...a cockerel and a pullet. After a fairly long dirt bath and nap afterwards, when they get up they are still in a crouched position like they can't straighten their legs all the way. They actually walk about in a squat for a minute or two and then begin to walk normally. What the heck? Can chickens legs fall asleep like ours do? I have no idea what would cause this.