Ah, that thread Alexis linked is interesting. Cubase always imports individual plugins when it finds new or changed files in your plugin folders. Because of how daylight savings time affects Windows, Cubase is tricked into thinking ALL plugins have changed, due to their timestamps being exactly an hour off, so it imports them all again.
So the rescan just happens due to a side-effect of DST changes in Windows on Cubase’s plugin quick-check code. It’s neither a feature or a bug, just an unlucky semi-annual side-effect. Something Steinberg could work around easily though, if they wanted to.