Why does this happen?

My sons lion head bunny just had three kits sometime since Wednesday. When I checked her nest box tonight, there they were, but all were dead they looked kind of squished. This rabbit has successfully raised babies before, but her sister had this exact same thing happen last year. Is there a reason for this? Am I doing something wrong?

Is her pen anywhere near, where something could have scared her? She could have squished them if something frightened her.