When the video was processing, YouTube, or a third party claimed that parts of your video, or all of the video contained audio-visual content which you do not own the rights to. The video is allowed to be uploaded I believe, as long as you disable all monetisation for that video. As Zeke has mentioned above, please check the video in the creator studio. Claims can be seen (and countered if you definitely hold the rights to the video) on your creator studio.