Apparently on imgflip if a meme has no likes and 2 dislikes it's supposed to get unfeatured (although there may be some exceptions?).
But anyway, my theory is that there are people who know this and go through and every time they see a meme with one downvote all on its own, they give it that extra downvote so they will have the feeling of power that they canceled a meme from being featured.
If this is true then it's probably just a matter of timing. Someone honestly doesn't like your meme, downvotes it, and then a troll comes along and gives it that second downvote.
There's other explanations of what it could be though. Either way it is frustrating!