This meta-post says the following:
Oh, and leave a comment explaining why you downvoted. With luck, the author will learn from this mistake and correct his post or write better answers in the future.
Can we add this to the FAQ somewhere? I get "drive-by downvoted" (just coined that term) sometimes and it's perplexing and frustrating. It just happened on a recent question of mine.
I searched the FAQ and see no mention encouraging commenting to accompany downvoting, and I'm not sure who ventures over to meta frequently enough to read a post like the one linked above.