I've rarely had it happen where their yolk sac is still intact and comes out with them. They should have absorbed it but for some reason didn't. This looks like more than a yolk sac and after hatching hundreds of eggs, I have never seen this. Perhaps it was something to do with a fume from...
Hiya, and welcome to BYC!! :frow
Could you post a picture of one or more of them? I've never heard of such a thing happening and doubt most have either.