Woah! This is a puzzler, definitely. Chickens tend to not get rabies, and an entire flock being diseased like that is extremely unlikely, I would think. I'm agreeing with the feed contamination or poisoning theory myself. Ergot from the feed?
What have you done lately that is a change, or different? Please think of anything possible, however minor it may be. Did you recently paint the coop, perhaps? Switch feed brands, or buy a new container to hold feed? Perhaps you have been working on a home renovation and it has nothing to do with the coop. Sometimes the answer lies in the most tiny, mundane details.