Oddly enough, I'm experiencing the same symptoms: a huge rear end and I waddle when I walk! Ok....sorry, I couldn't resist. Yes..this needs to be diagnosed if it's happening to so many chickens!!!!
Wow....I too have been having this problem!!!!!! A couple of weeks ago we had to put a hen down because her belly split open and it was all mushy inside!!! And now we have a hen acting the same way. We took her to the vet and he diagnosed her with synuvitis. (spelling?) She's actually full...