O.K. here is my theory and I might be onto something.
It is the first time I have tried to slump the piece upside down after the flat fire. (strips exposed to bottom and when slumping they simply peel away from each other. I just have never had this happen before because I always slumped with the 2 layers of clear that I flat fire and lay-up on being the buffer .. (bottom of flat fire is also bottom of slumping piece in the mold, usually, but this time the shiny side was DOWN facing the mold.) (actually the long rectangle piece and the birds nest piece also) both vessels split open on slump where strips are placed side by side.
P.S. another point is that the splits do not go right through either.