I kept reading in here about chicks falling out of nests and I couldn’t figure out what was going on. Then it finally happened to me. I let a hen hatch in a cat little bucket. Not a cat litter bin, but a bucket with top dimensions 7-1/2” x 11-1/2”. The chicks would climb up on Mama’s back and fall off. That nest was so small they’d miss the nest on the way down. That hen never abandoned the nest to take care of those chicks, I tossed chicks back in her nest four times, probably the same chicks. That nest was about 3 feet above the coop floor and those chicks were not hurt. I retired that nest right after she finished hatching. The regular nests I use are 16” x 16” with fairly high lips. Chicks don’t fall out of those.
Percheron, I believe you. I don’t know what was going on with yours or why they fell. Who can tell what decisions a broody hen will make when it comes to her chicks or nests. But that’s the only time I’ve ever experienced that problem.