It looked like the window was just drawn at its new position on each frame, without the previously drawn object being cleared by whatever was responsible for that (compositor, window manager)... I guess it meant that some OS processes crashed while the process drawing the window being dragged was still running.
I also think I remember that this often happened during Windows 98s notorious recoverable crashes, where waiting or mashing Ctrl+Alt+Del would sometimes allow one to kill the rogue program and/or explorer.exe in task Manager, sometimes not.
The more I think about it: I'd love a more knowledgeable explanation too!
https://archive.org/details/artcomputer00prue/page/185/mode/...
I guess you can't deep link to a non checked out book any more. Well, look and then return it I guess