Fused glass results soft edge

After fusing 2 standard pieces of glass (9X9 inch square), I went to remove the piece. This resulted in a corner breaking off. This corner didn't "snap" off. It fell off like a soft piece of candy. I used the same fusing program and glass from the same large piece of glass as a previous piece that turned out well!

 

Any suggestions as to what may have happened? Also, what should I do to prevent this from happening again?

Thank you.

Gail C