I'm really sorry to hear that, I feel pretty bad. I don’t know what happened. I hatch chick in elevated nests all the time and the only time I ever had a chick fall out was in a very small nest, a cat litter bucket that measured 7-1/2” x 11-1/2”. Chicks that hatch early often climb up on Mama’s back. When they fell off they missed the nest in that small nest and fell to the floor. It was summer and I tossed four chicks back in the nest with Mama before she finished that hatch and brought them all off the nest. That does not happen in my larger nests. They don’t miss the nest and fall out.
I don’t know what happened. I’ve found dead chicks before, but they were in the nest with Mama, not tossed out on the floor. I had a broody kill two of the eight chicks she hatched but raise the rest fine. I have no idea why she killed those two, but she pecked them to death. I don’t know if it was cold related or not for you.
If I were you with that experience I’d move them inside too.