Why do projects "tear" in the slumping phase?

I have been fusing glass for 15 years and run five to six kins a day year round. So one would lthink I have a fair understanding and grasp of what is going on here. However, without changing our firing schedule all the suddent we will have a rash of projects "tearing" during the slump firing.  Is it the mold coating wearing off? It is never all the projectsin the kilns just of course the most important ones.

 

Thanks for any help.

Puzzled