Each canning season I have a canning jar that the bottom cracks off during the canning process. Anyone know why? The jars are checked for cracks, chips and sterilized before each use.
Having the bottom of a jar pop off is a symptom of thermal shock to the bottle.
Don't place cold bottles into hot water or refill a canner full of hot bottles with cold water-- there is a good chance of cracking a bottle.
Another consideration, though in my opinion less likely to be the issue, is that your rings on your bottles should be only lightly tightened-- just enough to keep the lid on.
When the bottles are processed, the air in the head space expands and some of it pushes out past the lid. When the jars cool, the air in the head space shrinks and, since there is no longer enough to fill the space, it creates a vacuum that seals the bottle. If there is no head space or if the rings are tightened too much, the extra pressure in the bottle will not be able to escape and the bottle can burst.
I assume the bottom of your jars are not touching the bottom of the pan - you use a trivet. Thought I’d mention this, just in case.