What causes a request for a translation to be unpublished
Use of a machine translator (ie. Google Translate), perhaps?
1. Already done...published. Sometimes the requests remain open even though they have been done.
2. It may already have been already published before you requested it.
Always use 'Search For Artist Or Lyrics' box at the top of the page when submitting requests, or translations. Although this is not foolproof it may cut down problems.
3. Issues with posted lyrics/video.
4. Violations of site rules.
These are not all possibilities, but some to consider.
After your request is completed, it is unpublished automatically - logically, there'd be no longer a request after it being completed.
Requests are not often purposely removed, though - honestly, I've never seen something wrong with a request.