Infectious Brochitis will cause it.
There's no treatment. Birds do recover but may never be good layers again.
Have you noticed any respiratory problems with them?
We think one of our may have had it, which makes no sense really as it's an isolated case and the virus is infectious. And we didn't notice any breathing problems with her.
It's the only explanation we could find though.
It's usually vaccinated against along with the Newcastle vaccine, but it's not always effective.