More likely, I think, your thoughts about the size of my piece relative to the size of the kiln are the key. There was a lot of thermal mass at the edges of the piece with not a lot of room for air circulation around the edges. Despite what I thought was a conservative annealing schedule, I expect the center of the piece cooled faster.
It looks like a top fired kiln. I think you are right about the size of the piece and the lack of air circulation. The upshot is that the center of the bottom of your kiln is ice cold and the center of the shelf is cooling your glass too fast from bellow. You may be able to make it work by raising the shelf but the best thing would be to get another kiln if you want to do pieces this big. You need an element in the walls below the level of the shelf or an element in the floor.
I once fired a three inch thick piece three times before I finally moved it to my big kiln that has floor elements. Made all the difference.