I’ve replied this to another comment already, but it is probably this:

Most of the time it is due to Cross-seeding and additional or manipulated files.

I.e. the 99% torrent contains an additional file like an nfo or srt that was not in the original upload on a different site. People cross-seeding the original torrent don’t have that file, so you can only get 99% from them. And if the uploader of the torrent with the additional file stops seeding, there’s no one you can get it from.

Or if it is stuck at 99% without additional files, it’s possible that the file that was uploaded was manipulated. Stripped UID or other metadata that was changed. So there will be a mismatching piece that you can’t get from the cross-seeders. And again, if the uploader stops seeding the manipulated file, you can only get 99% from the cross-seeders.