Speaking generally, a site might be removed from a category either because it does't meet our
listing criteria or was unavailable for some time (in which case it might be deleted completely or left for a bit to see if things improve), because it fits better in a different category (in which case it would probably be moved to the better category), or because the current title and/or description needs improving to meet our
guidelines.
For the latter two cases, the site may be re-listed straght away with the problem fixed, or it may be left in the 'unreviewed' pool for someone else to deal with later. For a category change, the editor may not know enough about (or have permissions in) the destination category to be confident in listing it there. For the title/desc case, it might be that the reviewing editor didn't have time to fix it right away, or wanted to give the editor who previously listed the site a chance to correct their mistake (for example with a new editor who is being mentored by someone with more experience).
As others have said, you're not going to be able to find out what's happened to your site by asking here, but rest assured that human error and malicious de-listing are
extremely rare. Assuming your site is indeed listable (and you should be able to tell this for yourself by reading our
site selection criteria), I'm afraid you'll just have to be patient and wait, like everyone else who's suggested sites to the ODP.
