Chickens limping and eventually not walking

[Gvnam]

I've had two chickens (not full grown) start limping. The one started limping about a month ago and after a week it couldn't stand and could hardly move and I ended up shooting it. Last night I had another chicken with the same problem. I ended up having to shoot it too.

Could this be a disease or what? It shouldn't be the cold. They coop is plenty warm.