Why would babies die in the egg?

I just cut open two of my questionable eggs. ( I was 100% sure they were dead) Both had dead babies in them. One you could just tell it was a baby (this egg swelled up suddenly then shrunk and died) and the other was close to hatching still was connected to the yolk. ( clutch mate hatched 3 days ago). No obvious deformity. What causes this? This is the third time this has happened. I want to try and prevent this, next year.
Wondering if it could be my pair? They were rescues and they weren't being housed properly. They were already being housed together when I got them. I don't plan on breeding them next year.
My incubation temp is 84. Perlite is mixed .7:1 ratio. Eggs are marked and placed in deli cups with small air holes, 1 clutch per cup. I don't handle them after that. I do take the cups out once/ twice a week to weigh, replace water, or candle by putting a flashlight beside the egg.

Sorry to hear. Everything you're doing with the eggs sounds pretty good. Only thing I do differently is I keep mine with a .8:1 ratio and keep them in deli cups with absolutely no holes. I find mine dry out too quickly if they have holes. I've been too excited to keep from candling mine every week or two so mine have been taken out several times during their incubation to no ill effect

If the parents had nutritional deficiencies they could have been the cause of the babies not getting everything they needed to survive. They also could have had a deformity you could not see - such as with their heart, lungs, or something with their digestive system that prevented them from surviving once the support of the egg was gone. Not sure if they could have been too dry but I would reason that if they were moist enough to develop they should have been okay to hatch.