Why did you worm them this time? I've had chickens for 5 years and have never felt they were displaying symptoms that would cause me to poison them. I do add diatomaceous earth to their feed and garlic to their water.
Chickens can have several different intestinal parasites, depending on their environment and diet. Having a fecal run at the veterinarian's will help figure out if meds are needed, and what kind. Many flocks won't have issues, but some will need treating once or twice yearly. There's no one answer for everyone. Eating DE DOES NOT worm chickens!!! Cherry should find out what's killing those birds; parasites are on the list, but not a sure thing here. Mary