What you described sounds like they got Ergot poisoning. I had a similar scenario last year but didn't kill any of my birds.
My birds were having seizures, falling down, flopping and then just seize up. After a few minutes they would quit having the seizures.
At first I thought genetic neurological disorder but it wasn't just one species that was exhibiting this behavior.
The only thing I could think of was some type of poisoning. Had the feed anaylized at a lab and it came back positive for Ergot.
Ergot is caused by the fungi, Claviceps ssp. found in cereal grains. Cool, damp weather conditions allow the fungi to thrive in the growing grains.
I have my feed custom milled, and I have red winter wheat, rye, barley, and oats in my feed.
If you can and still have the feed you were feeding them, I'd have it tested.
ETA - This same poisoning is attributed to the Salem Witch trials, it affects humans the same way.