I'm not completely satisfied with the volume explanation, because do you see how distorted and crooked the elements look near the break? The rest of the element looks much better than that. So do the other 5 elements. There is another change I made to my kiln recently ... in January I bought new plate setters, which allow me to fire multiple plates in a very compact and dense stack. I always placed the stack right next to where this element broke. Hmmm. I know an uneven load causes your kiln to work unevenly. Maybe this stack of plates has a lot more mass than I realized. I should keep the stack further away from the elements from now on.
The good news is that because the remaining elements are only 55 firings old, it should be ok for me to replace only the broken element and its zone mate, rather than all 6. At $50 each, that's a relief.