From what I understand, in addition to the problem of the heat differential between the jar and pot, steam will form under the jar and cause them to bounce. The bouncing can cause the jars to break.

Glass to glass heat conduction is faster than water to glass. any difference in how fast the jar is heating will produce uneven stresses and possibly crack the glass where those meet. That's my theory.

Ive had jars fail because they were touching each other on the sides; but that i put down to mechanical battering.

There's also the fact that Ball jars have gotten thinner in the past few decades; after they became almost the entire food packaging industry.