I don't think I know. Sometimes chickens die somewhat mysteriously and the only way to know what happened is to have a necropsy done on them. Or to do one yourself.
Hi! :frow Welcome to BYC! So sorry for your loss. :hugs I don't think it was the rain as, here in the rainy state, my chickens get wet all the time. It hasn't killed anyone yet. They have coops and outdoor shelters to stay dry, but will still walk around in the rain. Silly birds.