Dead bluebirds

We recently had 2 fledgling bluebirds leave the box. Two days later we thought all the fledglings left but I heard young ones chirping in the box. Upon opening the box I found two dead male bluebirds that were fully developed. Upon taking them out of the box there were two more bluebirds that were much less developed and alive, and underneath the two dead ones. I feel there was no way the parents could feed the bottom two due to the dead ones smothering them. I had success in hand feeding the survivors mealworms for several hours. The next morning they were also dead. I would like your opinion on why the two birds were smothered by the two dead ones. The box is regulation size and we have had other broods in the box.