I have had some odd behavior with down-voting. I have noticed that just today, in a short span of time, around 6 questions that I have posted have suddenly been down-voted. Some of these questions have been around for a while, so this doesn't seem right. Also, when someone down-votes, there is no explanation why and I know it is not obligatory, but it would be nice to know at least for improvement. If someone is down-voting out of spite, then it is plain wrong.
There's a script that runs every day that'll check for unusual voting activity and invalidate it automatically. So generally, when you see something odd, give it a day and see if it clears up.
If it doesn't, you can always flag one of your posts and ask a moderator to look into it. Keep in mind though, on a site this small it's easy for folks to end up voting for each other a lot without actually trying - you've asked something like 18% of all questions on the site, so there's a pretty decent chance of someone running across your work.