They were sold as white rocks, but the sales lady admitted they were white rock hybrids with some Cornish in them. Is it possible that was the cause? I have never heard of anything like that. She insisted they were excellent layers. Naturally, I assumed that meant you could keep the as layers as well as meaties.
I don't think TSC puts much effort into training their staff about chickens. I like the people at my local TSC, but I would no sooner solicit advice about farming from them than I would ask them for health care advice.
It can be a painful experience, but you now know a lot more about chickens than you did then. It would make me feel a bit better to know their death was nothing I did, and that they lived out their natural lives entirely in a pleasant setting. Their fate in a broiler house would have been far, far worse.