Why are css styles falling off?

Question

css

I somehow open the Yandex web visor and, when viewing user actions on the site, I notice this: a person enters the site - everything is ok, moves around the site - ok, and then, at some point, all css flies. he [the person] continues to move on, and the pages remain without styles. there are many such people. all from different devices and different browsers. I, personally, could not catch this problem.

tell me which way to dig? what to see? Or maybe Yandex is deceiving?