What is root cause of egg binding?

Don't know if this should go in the injuries thread but here it is.

I've never had an egg bound bird until 2 weeks ago and 1 of my young flock became egg bound, I got her to release it but 2 days later it happened again and she died. A week later another from the same flock and she died. Today I have another.
All my layers get the same layer pellets, free choice oyster shell, occasionally AVC in the water. Few treats - usually a wild bird mix or oats. Is there something that may be causing this?
They're black Penedesencas that started laying at 20 weeks and they're 28 weeks now.
No problems with 2 other flocks.
Thanks