mottles blown out crusts

I have been trying to bake the Dark Rye Bread in the Laurel's Kitchen Bread Book.  I have tried three times now and have been unsuccesful.   This last time, I thought I had succeeded until I looked back into the oven and looked at my loaves during the final proofing.  They had cracks blown out all over them.  I tried several more times to fix them and was unsuccessful.  I don't understand something obviously.  What am I doing wrong to have the tops do this?  I do fine with the Yogurt Bread (whole wheat) in the same book and Maple Oatmeat Bread (whole wheat) that I came across.  I know that rye is difficult to work with and I am an amateur.   It can be wearisome to go to such efforts and have these results.