Dunting question. Are there many reasons for dunting cracking?

Recently I made several repairs on my electric kiln (older Cress, with kiln sitter) including changing out all of the elements and replacing a relay. The first firing (cone 04, bisque) completed beautifully. The next firing (cone 5, glaze) cut off before completing the cycle, leaving the pieces half done. One of the new elements had melted at the terminal (probably a loose connection – very expensive and time consuming lesson).

Refiring the pieces, a few had serious dunting damage. Lid was opened after total cooling. Pieces were on different shelves, with a variety of glazes, and a couple of different clay types, all cone 5/6. For many years this has been a workhorse of a kiln.

Any ideas? Other than gremlins!