I found this phenomenon happened when I edited this post. It was converted to "community wiki" automatically. And I did a little search and found the explanations in this post "What are Community Wiki posts?":
Posts enter community wiki mode when one of the following happens:
The body of the post has been edited by at least five (5) different users.
The post has been edited ten (10) times by the original owner.
The answer's author checks the community wiki checkbox when composing the answer. Note that this checkbox isn't available to new users. The checkbox also is not available if the question is already a Community Wiki.
Since October 2010, there is no community wiki checkbox when asking a question.
The answer is edited by its original author, who when doing so opts to check the community wiki.
An answer posted to a Community Wiki question will also be Community Wiki. Note that when a question is made CW after answers have already been posted, the existing answers are not normally converted to CW automatically.
The question generates more than 30 answers (15 on Super User, 60 on Code Golf). In this case, the question and all answers will enter community mode, as will any future answers.
A moderator has reason to believe that the question serves better in community wiki mode - if you believe your question should be converted to a Community Wiki, you may flag it for moderator attention.
This confuses me very much. If one wants to earn reputations on their answer, one has to not revise the answer more than 9 times. Doesn't it discourage people to revise their answer?
Personally I don't hope my answers become community wiki because:
Basically I want reps from them;
As "a much lower reputation (100 instead of 2000) is needed to edit a community wiki post", I really don't want some careful answer can be edited so easily without peer review.
Although that post also introduces a way to remove the community wiki status:
... if you feel your post has been unfairly converted to community wiki and deserves special treatment, you can flag it for moderator attention and plead your case to a community moderator.
Still, I guess everyone that hasn't intentionally mark their post "community wiki" would feel they should "deserve special treatment" (though they might give up the right to flag it for moderator) when their answer is automatically converted into community wiki. Wouldn't this create more work for moderators?
BTW, would the moderators please remove the "community wiki" status on my two posts? Thank you! (It seems this "Removing Community Wiki Status Application" process hasn't been put into the help documentations. Next time I will try the more proper "flag" way.)