Peter R. wrote
What would be the most likely reason a cylinder in a high-time engine
would crack, old-age or poor temperature management on a descent?

While both are possible, neither is most likely.

When considering what causes an aviation problem, you need to consider
what aspect is most under the control of the FAA. That will be the
biggest problem.

In this case, the FAA controls design (through the awarding of a type
certificate or STC or writing the TSO) and manufacturing/quality
control (through the awarding of a production certificate or PMA) so
those are in fact the two most likely reasons for cylinder cracking -
poor design and poor QC.

Michael