Taken from the offcial Steemit FAQ:

Does a downvote mean that I did something wrong?

Just because you received a downvote does not mean that you did something wrong. The downvoting person may have just been voting to reallocate the rewards in a way that they felt was more beneficial to the other active posts in the platform. Often users will leave a comment explaining why they downvoted, but sometimes they might not. If they left a reason, it is up to you to determine if you did anything wrong, and if there is anything you want to change.