It is not the kiln. These are stress fractures -- could be the result of uneven heating/cooling or the forms getting snagged on an uneven kiln shelf surface during expansion/contraction during firing. They can also result from construction of the ware.
Getting them up off the kiln shelf -- using sand, grog, cookies, coils, slats, etc. -- is a good step. That will give the form a floating surface for the expansion/contraction that occurs during firing and more even cooling. Should this happen again, notice if the cracks are starting from the edge closest to the elements or to the center of the shelf. When you have a firing issue like cracks, take a second and photograph the shelf before unloading so you can figure out what happened. How densely is the kiln loaded? Are the shelves close to each other or do you have some spacing for air circulation -- the shelf above will also heat the items below. And, are there rough spots on your kiln shelves where a flat item could get snagged as it expands/contracts during firing?
And, as Benzine suggested, work on reducing/smoothening sharp notches . . . a place where most cracks start because folks tend to overlap their cuts and the deep part of the notch is not a strong. A good method is to use a needle tool or nail to make a hold at the point where the cuts join, then use your fettling knife or x-acto knife to cut to the hole. The round of the hole will make that a stronger notch.
A word about putting grog (or sand) on your kiln shelves. I found that it doesn't need to be more than a thin coating.
The easy way to get an even thin coating is to hold your hand about a foot or more over the shelf and sprinkle as you move
over the entire surface. Of course you are far away from anything that doesn't need grog on it!
Since putting grog on my kiln shelves, I have had no cracking.
I would likely go with adhering post firing; that way, if one layer does crack, you can choose to either use it or discard it. And, until you get the cracking figured out, I'd play it safe . . . especially since the layers are all flat. Stacking in the glaze firing will add weight and bulk to the items and that can affect cooling.