The engines of the 737 MAX$^1$ and Airbus A320neo family$^2$ are [more] prone to bowing (shafts deforming if no debow is applied during start – as I understand it, this happens in a turnaround while the core is still hot).
The debow solution is cold motoring to harmonize the temperatures of the various engine parts. This in turn leads to much longer start durations (was upwards of 7 minutes on the A320neo until a dual-cooling button (shown below) was added to allow the motoring of one while the other was being started, which brought the delay down, but still takes longer than the ceo).
Operational impact aside, why are those engines prone to bowing? The only other jet-liner I'm aware of that needed a debow procedure was Concorde.
My first thought was the higher overall pressure ratios (OPR) of the new engines, while plausible, Concorde's OPR on ground was nowhere near the levels of the new engines (15.5 vs. 40).
What design element/changes made those engines require the noticeable debowing procedure?