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Or simple mechanical stress when the ISS raises its altitude. The thrusters cause vibration, and are not applying a uniform force across its surface.

I was curious so I checked what temparature the sun exposed parts reached and what the shadow side reaches; 121c to -157c a temparature difference of over 278 degrees. Your remark was almost spot on.

https://science.nasa.gov/science-news/science-at-nasa/2001/a...

The hull where they're able to manually inspect it shouldn't be going through quite that extreme of a temperature differential because it's wrapped in insulation and protective layers. But it's also almost 21 years old so even the smaller actual cycle is probably getting to it combined with all the other mechanical stresses from equipment and thruster firings.