It can be a combination of too much heat in the mold & your recipe. I have a recipe that cracks no matter what. However, now I elevate my mold on a wire rack so air can circulate all around it. I also don't cover my mold at all.
That recipe now only has a fine hairline crack that closes as the soap cools down, but it always cracks, so I think it's the recipe.
The other recipes aren't cracking at all.
If you're using a TOG mold that comes with a liner, you'll find it insulates like crazy!!