I had a Home Depot bucket turned inside down in the run. Last week, a girl came up missing. Because we have coyotes all around, I assumed one slipped around and got her when we were all busy around the barn. Today to my dismay, I turned over the bucket and she was under the bucket dead. How in the world did she get under it?
We had this happen this summer although it was a feed dish. 1 of my barred rock pullets died when the dish flipped over on her. I think it just happens. Standing in the wrong place at the wrong time.
. My guess is that the bucket got knocked over, she got into it, panicked and turned it over on herself. I have tried to anticipate any weird thing that can happen with my chickens, because they will find a way for it to happen. I always secure doors, any objects, hang buckets, cover up or :/bend nails, pick up glass, paper, plastic, or objects that appear in the dirt. So sorry for your loss.
I had a pullet get caught between a horse water bucket and the growout pen fence. She had been missing for 3 days. She was dehydrated and weak, she couldn't walk for a while. But, she eventually got straightened out. She lives with a little girl now.
Don't beat yourself up, similar things have happened to many of us. Check your property for things they can get trapped in/under like buckets, tarps, tight spaces, etc. Poorly stacked bales of hay can also be hazardous. -Kathy