Seems like the accounts that made each downvote would have to be stored somewhere, since anyone can go back and change their votes at any time. So it should be easy enough to mine the database for curious patterns if you knew what you were looking for.

Paul Graham at Hacker News has hinted that he has all sorts of hidden machinery in placing looking for odd voting activity. Given how big and occasionally ugly it has gotten, Reddit probably also has all sorts of inside workings going on, though I don't know how open they are about them.