29 dead chicks from Murray mcmurray, why?

So… you may remember me. I had ordered 29 chicks from Murray mc Murray and thought I had one survivor! She came in Friday and most of them died by Saturday night. She was eating, drinking, moving around, and seemed happy… then I wake up today, Wednesday morning, to her limp and gasping for air just like the rest right before they died. She was convulsing in her head occasionally and gasping, unresponsive. I kept her warm and tried to keep her neck mostly straight while she kept moving. She stiffened and passed in my hands.

What caused this? I have 8 1-2 week old tractor supply chicks thriving in the same conditions, and 6 other tractor supply chicks who are now a month old and I had only one loss. Why did I lose 29/29 but kept the other 13/14? I did some searching online but it all said respiratory infection- or just “failure to thrive”. Failure to thrive in 29 chicks? But also, respiratory infection in 29 chicks? Could it be from the shipping conditions? Shipping temperature? They came in a smaller than usual box because it is cold outside, Murray mc Murray standard, but at least 12 arrived dead. Some looked like they’d been dead… a while. Did this cause the infection?