With all those jars cracking even though you could hold your hand in the water, it sounds like the jars had direct contact with a metal pot that was used to heat the water. The pot will be much hotter than the water, and may also retain heat longer than the water.
To avoid the cracking problem in the future, heat the water in one pot, then transfer it to a different pot and place a small towel or a couple of paper towel sheets on the bottom of the pot so the jar never contacts metal.
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