@ChickenSuicideEmergency I'm sorry for your loss.
I can't imagine how upsetting it would be to watch all your birds die.
We can speculate all day long and still not come up with the correct answer for you. The only way to know for sure would to have had testing performed. Since you burned the bodies, then sending a few birds for testing would be out of the question, too bad - this would have been a very good way to get some answers.
If you suspect feed may be the culprit. Collect several samples and send it to your state lab for testing.
You don't mention your location, so you can look up your state lab here:
http://www.metzerfarms.com/PoultryLabs.cfm
If you can't get testing and you feel your birds may have had a contagious disease, clean out the coop, sanitize with Virkon S or Oxine, wait several months before re-stocking.