Any user can choose to decline payout on a post. The reasons vary, sometimes it's simply a community update. I once declined payout on a post that highlighted the charitable deeds of a friend, since it didn't seem appropriate to profit from it. Hope this helps clarify it a bit for you. :)

This post may help you better understand more details of the post payout process as well...

Link: The Ultimate Guide To Steemit Payouts