What causes rake uploads:fix_relative_upload_links

I just did an import from smf2. It worked fine on my machine but when the client ran it, a bunch of images got tombstoned, and recover from tombstone says to run

 rake uploads:fix_relative_upload_links

Does anyone have a guess what might have gone wrong?

And now a bunch of images are just failing to render. That is, something like


is rendering https://host.com/images/transparent.png