I didn't downvote that question, and I don't see a obvious reason to downvote it. You left some junk "NB>" characters in there, but that relatively small editing oversight doesn't rise to the level of deserving a downvote in my opinion. I upvoted it to compensate for one of the downvotes.
To answer your question, there is no real incentive to downvote other than the desire to keep the site clean. We get a lot of sloppily worded questions with baby talk instead of real English words, and I downvote those regularly, for example. You don't gain or loose any rep by voting on questions. I guess there is a very weak incentive in that votes on questions count towards your total votes cast, and I think there is a badge or two when you get to certain levels. However, badges don't mean much here and I doubt anyone is going around randomly voting to gain a badge.
Sometimes downvotes happen for mysterious reasons. I still think this site would be better off if all votes were public, but those that run this site disagree. I don't have a problem with someone politely asking why they were downvoted, and I have done that myself (usually don't get any answer), but calling the downvoters children without knowing their reasons is just as wrong. That in itself can be deserving of a downvote in my opinion.