No obvious injuries. I'm thinking the stepped on theory is the most likely.
I admit I went out a lot and watched her. I'm not going anywhere near her for the next 2 days. I wonder if her agitation at me caused her to step on it. But there were 11 eggs, maybe it is just crowded. I tried moving her and the eggs very briefly 2 days ago to a brooder area (that is what I did in May, the last time she went broody & it was fine) but she didn't want to stay so moved them all back, very carefully of course!