What Happened - A Frustrating Result

Well, here is a really strange result.  I created a strip-cut blank and a incorporated some of the murrine tiles I made a while ago.  It is all made from Bullseye glass so I'm pretty sure it is all compatible.  I spent several hours cutting the strips, alternating clear and opaque ones which looked really nice when setting up the layout.  I used murrine that had clear glass centers surrounded by an amber brown color.  These made little windows or stepping stones across the piece.  

I fused the blank in a 12" ring.  It came out perfectly - just as I expected.  I got some new slumping molds for Father's Day.  One was the deep sided bowl.  This is a 12" diameter mold across the top but only a few inches deep.  This should have worked perfectly in my small kiln.  In a nutshell - IT DIDN'T.  

The picture above is what I saw when I opened the kiln.  I peeked when the piece was heating up to make sure everything was OK.  At a little over 1000 degrees (F) it was perfectly fine.  It was late so I went to bed, around midnight, knowing that it would take most of the night and the next morning to slump, anneal, and cool.  

A couple of notes, I had a good kiln wash on the mold.  I was happy and it was uniform.  Note that the glass came out perfectly fine from the mold and only had a slight dusting of kiln wash residue which wiped right off.  The blank was perfectly centered in the opening and was very level.  The mold has an indented bottom on the outside so I didn't think it needed to be up off the kiln floor.  The only other thing which is a little unusual is the placement of the thermocouple.  It is about 3/8" from the side of the mold.  It isn't touching the mold but its placement meant I had to keep the entire set up slightly off center in the kiln.

Here is a more close up picture of the break.  Note that the piece seems to have slid down the sides of the mold unevenly as one side is higher in the mold than the other.
 
 
I used the following schedule which I felt was conservative.  I peeked in the morning when the temperature was about 750 and the piece was cracked.

Step   Rate    Temp (F)   Hold
  1      100      1000         60
  2      240      1240         30
  3      9999     900          180
  4      50        700           0

The picture below is a close up of the crack from the underside.  Note that it is very smooth and fire polished.

 So I'm really at a loss to explain why this happened.  It isn't a compatibility issue that I can see.  All the white, red, and clear strips where full length strips so there was no piecing together.  Seems to me that since the edges of the crack are smooth that the piece cracked sometime after the 1000F mark but before the full fuse.  

Help?  Thoughts?  Ideas?