I think the most common way for it to happen is if another user "flags" your meme (e.g. for being against site guidelines), and the mods agree it's a bit questionable, they may bump your meme back to submission.
Or the mods can use their privilege to bump a meme back to submission at any time for whatever reason they want (e.g. if there's an extra-bitter political fight developing on that meme), but I don't think they do that very often.