1. You loaded it up too fast and not in even layers so that you're getting uneven shrinkage.
2. The clay has been applied loosely and is cracking along the joints.
Also, what kind of clay are you using, normally in automotive design the clays most widely used are Chavant and Faber. If you are using a water based clay then that will definitely crack when it dries out. As you said, to "bung" on the clay is probably the reason why it has cracked.
To rectify the problem you will have to cut the clay back to the base of the crack and re-load with clay. Heat the area to be loaded with new clay so that it blends in with the old. Make sure to push the clay on firmly and this should repair the damage.
