It actually is not uncommon. After the crop fills to capacity the chick will no longer be able to lift its own head. If the chick can not lift its head the parents can not feed it. Dead in the Shell is exactly like it sounds. The embryo dies at some point in development and fails to hatch out of the egg.
Bacteria can cause an embryo to die at any stage of development.
The shell itself is full of small holes which allow moisture, air and even bacteria to enter. This is why nests should be cleaned out thoroughly or replaced between clutches. Low humidity in your bird room can also cause the chick to die shortly before hatching. Without proper humidity the fluid protecting the chick will dry and become sticky. Sticking to the shell and not being able to rotate will trap the chick inside the egg and it will die.