Could this bird have eaten some moldy feed?
Totally guessing here.
https://pubmed.ncbi.nlm.nih.gov/14995975/
https://extension.psu.edu/aspergillosis-in-poultry#:~:text=Clinical%20signs,silent%20pneumonia%20in%20the%20past).
Helpful hint.
If you need to open a bird up again in the future, it's best to just cut along each side of the spine to open it up and then you don't have to worry about cutting up the breast meat to get to the innards.