Currently, there are only two ways an answer can become community wiki:
- the author can do this by checking the community wiki box that appears below the post-editor box.
- a moderator (or some other diamond-
encrusted -entrusted user) can manually convert it to community wiki at any point.
(This has been the case since all automatic CW-conversions were removed from the system in April 2014.)
So, yes, a moderator did convert your answer to community wiki.1 This action and also the declining of your flag are likely both rooted in an uncertainty about what to do with answers of this sort. Of course, the answer you gave is likely the answer sought by the asker, which is perhaps the crux of the issue. I think this uncertainty is felt fairly widely in the community, and feel it pretty strongly myself. I am coming to be of the opinion that the real issue is with the question.2
But as this really doesn't concern your (or anyone else's) answer, the decision to post it as non-CW should have been respected.
1 Up until community wiki was removed from the answer this action was visible in the revision history. Now it is not, and is even hidden from the moderator timeline of the post. Very odd, if you ask me.
2 For a pretty good (IMHO) exposition of this viewpoint see Raphael's answer on meta.cs.SE.