I would bet that it has to do with a very overlooked issue with incubation. This is the fact that chicks, at around five days before hatching, are actually breathing. They put their beak into the air sack in the egg and breath.
Turning an egg at this point is a big no. This is well known for duck eggs, but for some reason is overlooked in chickens. Turning after this point means that they have to reposition, or the air bubble will move on them and they'll suffocate. (air is absorbed into the shell through little pores through the entire incubation process).
The other factor in their breathing is that they are usually locked into a rather well sealed box, especially with still air incubators. These boxes are sealed to keep the heat in, but then when the oxygen levels are vital, there's no air. So, they can suffer oxygen depletion.
Ask yourself, would you stuff eight hatched chicks into that box and seal it? If not, then there isn't enough oxygen for chicks who are about to hatch. They start breathing several days before they hatch (and you can tell this by the fact that they'll even chirp in the shell several days before they hatch, or by candling which will show the beak poked into the air hole).
So, about five days before hatching, Stop turning. And open the incubator several times a day to let in some air.
Sorry for your loss, and hope this helps for future babies.