A user had put a bounty on this question. The bounty was later cleared by a moderator, who sent the user a private message citing the reason for revoking the bounty (supplied by the user in chat):
We have cancelled your bounty on [the question]. Now that the issue you were discussing in chat (presumably why you started this bounty) has been cleared up, we see no need to keep highlighting this question. The original question was resolved a year ago.
As far as I understand, bounties can be freely applied to any eligible question by anyone with the required rep for any reason, and there is no explicit policy basis that says that bounties can be revoked simply because the moderators feel that there is no need to "highlight" a given question. (If there is such a policy, it would mean that bounties intended to be awarded to rather old existing answers wouldn't be permitted, but I see those happening and they don't get cancelled.)
Was there some different reason why the bounty was cancelled? Did the fact that they selected the "draw attention" reason play a part in the bounty cancellation? Does the user have a history of abusing bounties that I'm not aware of (hence the mod message warning)? Or was it just a helpful move so that the user doesn't waste the rep?