I generally see more birds dying during spring and fall as the temperature shifts can be stressful on them. I keep a larger flock, so I can see a few die each year. I have had a few die closer together. This spring I have already lost two as well, one was 8, the other a 2 year old.
If it's a...
About half my birds will just die one day, appearing fine the day before. I generally assume it's heart problems, or something like fatty liver syndrome which can kill quickly. That's from having too much internal fat.
https://www.thehappychickencoop.com/feeding-your-chickens-to-death/