.... one of the Dove eggs was cracked on the bottom of the cage.
They may have accidently dropped it, or sometimes they know an egg is infertile and kick it out.
They can do it because if they sit on the eggs, get off, then get back off, an infertile egg will cool down much quicker than a fertile egg. So they will kick out an egg if they think its infertile.
Don't count your doves before they hatch ;)