What has happened is that bacteria has gotten inside the eggs. The egg material inside is a great food for bacteria. Feeding them egg is a common way used to culture bacteria. Incubation temperature is a perfect temperature to promote bacteria growth. Once bacteria gets in it multiplies rapidly. It doesn’t matter if the egg is fertile or not, developing or not. That bacteria will grow so fast and produce gas so it either oozes or explodes.
When a hen or duck lays an egg, the last thing she does is put a coating called bloom on the egg. That’s why they look wet when they are first laid. It quickly dries and forms a real good barrier on the egg to keep bacteria out. It’s not perfect but it does make a huge difference.
If you remove that bloom then bacteria can enter through the porous shell, so don’t wash the eggs or scratch the coating. If you handle the eggs with dirty, wet, or oily hands you can remove some of the bloom, so keep your hands clean. If the eggs are dirty, especially with a glob or poop, that can let bacteria in even if the bloom is intact. A bit of dried poop or dirt isn’t too bad but the more you have the more dangerous it is. Sometimes when an egg is laid and the bloom is still wet you can get pieces of bedding stuck to the egg. If you scratch that off you may remove bloom. I do that anyway if it’s not too bad and don’t have a problem. If an egg breaks and you get egg goop on the other eggs, which occasionally happens under a broody if the egg shells are pretty thin, the other eggs will usually be ruined. Egg on the shell is really bad. Don’t set dirty eggs and try real hard to not take off the bloom.
You still need a source of bacteria to get inside of the egg for this to be a problem. If there is no bacteria around it can’t get inside the egg. You need to sterilize the incubator really well each and every time you use it. Some people wash eggs before they incubate and generally do OK but a dirty incubator is a real risk. Of course try to keep them clean before incubation starts.
I don’t know what specifically caused bacteria to get inside your eggs but you are right to remove them. Hopefully this will give you some clues as to what might have happened. Good luck with the rest.
My incubator was brand new, I guess I didn't even think about washing it!
I had trouble with my entire batch of 14 shipped Cream Legbar eggs. Only two remain, and one of them is the oozing one. Most didn't develope, several had blood rings. The Bielefelders I'm hatching at the same time (shipped from a different breeder) only had two eggs that didn't develope (out of 12).
Should I be sanitizing my eggs somehow? I've seen spray on the internet for hatching eggs, but assumed it may take the bloom off.