For me, they were drm free ebooks I bought legitimately and had to sideload via calibre because the kindle doesn't support epub. It's an old Paperwhite, and it only happens when I go out of airplane mode. However, I have to go out of airplane mode (or had to before I ditched the kindle for a kobo) to get library books and the occasional actual Amazon kindle book when I didn't feel like waiting for the library to get me to the front of the list.
*Edit to add: I believe the kindle won't delete epubs you sideload via the Amazon kindle email gateway, but I have no interest in doing that.
*Edit to add: I believe the kindle won't delete epubs you sideload via the Amazon kindle email gateway, but I have no interest in doing that.