Concerted, not necessarily organized. It's possible for one person to put a pretty big dent in someone else's karma if they're tolerant of boredom and have a reasonable amount of karma of their own; you get four possible downvotes to each upvote of your own (upvotes aren't capped), which is only rate-limiting if you're new, downvoting everything you see, or heavily downvoted yourself.

This just happens to have been a sensitive issue recently, as the links in JoshuaZ's ancestor comment might imply.