I'd like to know why this has been voted down.

Most likely it was one of the one or two people who systematically downvote a large number of comments. As I've mentioned before, there's at least one user who appears to periodically downvote all my comments, ever since I first ended up on the top 10 contributors list.

It's kind of weird to watch, because my karma will drop by dozens of points in an hour, and I can see which comments were voted down. Then, most of the comments that are on active posts get slowly voted back up above zero. Then, sometimes the second downvoter comes through and knocks the marginal comments back to zero or negative. Depending on how old the comments are when the second wave hits, they may get "left behind" and not re-upvoted by readers.

The silly thing about all this is that the downvoters are simply teaching me to ignore any negative scores on my comments, especially if the comments fall just outside the trailing upvote window created by post activity.