The "why" is thermal stress; like most materials, glass expands and contracts as it gets warmer and cooler. A jar is likely to break during these changes if it has any cracks, chips, or flaws, even tiny ones too small to see.
Warming your pressure cooker more slowly might help. Inspecting your jars for imperfections might help a little. My mother's average loss was a jar about every third batch, but she never could eliminate all breakage.
It's also possible the lids were tightened down TOO tight. The year my husband put the lids on for me I had a lot of breakage....my version of 'tight'and his are very different and it is hard to explain...I was water bath canning but I think the same would apply to pressure canning The ring needs to be screwed on tightly but not so tight that no air at all can escape...My son had breakage in pressure canning also and I suspect that the rings were too tight then also. and then you don't want them loose enough that the liquid or food particles that you are canning escape.....
EDIT...I meant to ask ...what are you canning?
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Jars are thinner these days, losing one per box of new jars seems to be normal
Of course, those old jars already lost the weak ones years ago so it may just be the way it always was...
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My canning is much more low tech than yours, but we get only about one jar breaking in 100.
We re-use commercial jam and pickle jars with common one-piece lids. We do buy new lids, but also re-use old ones if they are in good shape.
When one fails to seal, it often turns out to have had some flaw like a chip in the rim or dent in the lid.
Some years we get more breakage, especially in the juice bottle we refill and close with a clamping lid press thingy. Supposedly the recycling junk pickers can tell whether those bottles are good or bad by tapping them and listening to the ping. I haven't tried it. We are just doing a boiling water bath in a big flat bottomed pot, no special basket or anything. If we seem to be getting more breakage, we lay a cloth or towel or old sack in the bottom of the pot so the jars or bottles don't clink quite as much. Seems to help, but of course I can't be sure how many would have broken in that batch without it.
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Two or three additional points, on jar breakage and also seal failure:
--Supposedly one must never use a metal implement, especially a sharp edged or pointed one, to pack food down or release air bubbles from jars when filling. These tools can leave nearly invisible, hairline scratches on the glass, where cracks are triggered to form later from whatever direct cause.
--When pressure canning it is vital to exhaust the canner, then bring it to pressure and keep the pressure even the entire time required to process. If the pressure fluctuates too much or too dramatically (such as by canning over a campfire or trying to multitask too much----I have done both!), this can cause failures. When processing is done, the closed cooker must be allowed to depressurize of itself, without opening before the gauge reads zero or trying to hasten the process with cold water, etc.
--This is a suspicion of my own that I haven't read about anywhere and is on the cutting edge of my own trial-and-error research....."exhausting" the canner means to allow the canner to boil with the valve open until a steady stream of visible steam is seen to come from this for the allotted time (usually 7-10 minutes) before closing the valves and bringing to pressure. I think this process of replacing all the hot air inside the pot with steam is made more difficult if the canner is only partially full of jars.....it stands to reason that the more empty space there is in there the longer this will take and the more likely it will be incomplete. So I've made up several dummy jars full of water and from now on will fill a partially full canner up to full with these.
The lid tightness has some effect on breakage. The lids/screw bands are not supposed to be on there super tight. During the heating/canning/sterilizing process, the contents expand, and some of the air/steam/liquid may exit the jar.
That's ok and expected and possibly even required to get a good vacuum seal.
So don't overtighten the lids.
And, as noted, try to minimize the thermal shock by pre-heating the jars, the stuff going into the jars, to reduce the temperature delta.
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