I have several barred rocks and out of all my chickens, now that I think about it, have the best survival rate. I haven't lost a single one in 2 1/2 years.
I've lost at least two of my chickens from that, it's a sad thing. But it happends. Were your chickens overweight? That may be a cause, especially in younger hens. But my hen that died from it in early December was almost three years and about 5 pounds. So I knew it wasn't an illness.
One of my very first hens died in june or july 2007, about 16 months old was a big girl died supposedly from that. I wasn't home so I couldn't know the cause of death. I woulda cried back then.
You probably should wait a while if you ever decide to get any more barred rocks. I have the worst luck with fancy breeds. The closest I have now to a fancy is 2 Wellsummer hens and 15 or so americaunas.
But just listen, it happends. I lost my first chicken to my dog when I was ten(hated her ever since) and I cried for three hours straight. And only came out of my room to close my girls up.
She was a black australorp and my favourite out of my first ten. And six years later, I only have five left. One of each breed. RI red, barred rock, buff orphington, Black Australorp, and one Americana. Americaunas don't get as big and bc they're usually brown they don't get spotted by hawks too easily.
The second to die was killed my my dads dog(after the first chicken out of like 10 she killed she was dubbed my dads dog. Killer dog # 2) that hen was the best dang hen I have ever had. Second favourite. And people wonder y I don't like dogs.
Third was the buff I mentioned. 'Course I've lost so many chickens since the first death, I don't cry about it anymore. I usually mutter a few curses and bury the bird. I haven't lost any exceptto hawks every 4-6 months and killer dog #1got attacked by my muscovies and she won't go anywhere near anything with feathers.
I'm sorry if I depressed you or anyone who read this... I just learned what to do and what not to do from a lot of trial and error. Loosing a bird right after it starts laying sucks big time. And even more if you were as attatched as I was. I cried away all my tears on my birds a while back.
Don't let me depress you, just try a different breed. Not all the heavies you get will die like that.