I can't look at your ratings since you didn't include the actual URL to your ratings page so if you are absolutely positive that you didn't vote for those titles, I can only hypothesize that what happened here is that someone else voted for them using a computer/browser where you had logged into your IMDb account but never logged out.
If you log into IMDb on a shared computer or device and never logout, anyone else using the same browser (friend, relative, housemate, co-worker or a stranger, if you were using a public terminal) can cast votes and perform other actions using your account.
You may want to check if all those votes you don't recognize have been cast around the same time, which would tend to confirm that it was done by someone in a single session using a logged-in browser (to see the vote dates, use the 'export' feature on the ratings page (go to
http://www.imdb.com/list/ratings and click on 'Export this list' -- the resulting file will have a column with the dates).
And you should definitely change your password and make sure you log out of any shared/public computer once you are done using IMDb on it.