Unless there was something wrong with the conditions the eggs were being kept in (high temperature spike, for example), it might go back to one or both of the parents. When I think of eggs dying early in incubation, I think of miscarriage. I know the process is very different, but there are similarities too and with miscarriage it often goes back to the fertility and reproductive health of the parents.
I use 4" flexwatt under the tubs in my rack and my thermostat is set at 91F. The temperature in the tubs is reading 89F at the warm end and at 78F at the cool end. Not sure if that gives any insight at all, but it's what I'm working with. I prefer keeping things a little cooler when I can because the room the rack is kept in can heat up quite a bit during the summer months if a certain someone in my household decides to be cost-conscious and turns the a/c off while we're away.