When the shared link page is loaded, it creates a time-sensitive links, under the covers, for each file on that page. If enough time elapses since that shared link page has been loaded in the browser, the file links on that page will expire. If you refresh the browser page (click the refresh/reload button), new file links will be generated.
If there are several large files on a single page, users can run into this issue since it takes a long time to download all of the files.