If you own a playlist and have set yourself as (the only) collaborator this playlist is also shown under "Collaborations".

This seems confusing, it should be only listed as a Collaboration if it is actually shared with another user.

This happened to me when saving playlists from the recommendations. Recommendations by design seem to be shared with the user they are created for. Hence my recommendation playlists have "outsidecontext" set as a collaborator.

When this got saved to my personal playlist I became the owner of that playlist but were also listed as collaborator. An example is:

https://listenbrainz.org/playlist/f75a4e4f-2bd2-47be-ab1a-057db3e7ceb8