I am thinking that since it only happens in the clutches that I incubate, its more that just a random event. There is very little development that takes place so late in the incubation, mostly its just growth at that point. Also, I doubt an unexplainable birth defect would mysteriously take out 25% of a normal healthy population. This is happening across numerous females in a variety of incubation conditions. I am pretty confident there is an underlying reason.

When you say "quite common" what do you mean? With your pythons? How common?