Glass crack during slump firing

I am making a set of 8 inch plates for a customer, using the 8 inch swoop mold.  I used kiln wash on the mold, and also covered it with fusing paper, and I just checked it and found in cracked in half.  I use a JenKen Bonny Glo kiln and follow their program instructions in most every piece I do, tweaking some programs as necessary.  This piece was initially tack fused and had no cracks when I finished that procedure.  Any assistance as to what caused the problem and how I can remedy it occuring again will be appreciated.  Please email any responses to beachbumv4@hotmail.com.

Thank you!!