1) I am canning over a campfire. The heat is difficult to regulate this way, and extreme heat could contribute.
2) The jars that have been breaking are old and have been used many times over the years.
3) only the older Kerr jars are breaking. My Mason jars, which are older, have faired just fine. None of my Ball jars and no newer Kerr jars have suffered.
4) No cracks, chips, or deep scratches are obvious.
I used only Mason and Ball jars the past two nights, and only the Kerr pint jars, and I am happy to say that there was zero failures.
Your guess is as good as mine. I think the combination of age, and extreme heat are the main culprits. And possibly just a bad batch of glass.