As you have discovered some sebright hens do go broody and are generally good mothers. Any female chicks from that hen are also more likely to go broody than chicks from a non broody hen.
We think it's going under there because it's afraid of our bigger rooster. We also think it's going under there to lay eggs, we started to think about that because it would only come out to eat. It may be hatching eggs but we wil never know until it happens.
I'm new to this sight too. I had a question about Sebrights. Are they cold hardy. I have seen some posts that said there's had no problems with cold. I wonder if they would go inside their coop if they were cold.