Your 12th edit made it so automatically, see the [revisions](http://stackoverflow.com/posts/3316970/revisions). For further explanation see [Why did my answer become community wiki, history says I did that, but I didn’t](https://meta.stackexchange.com/questions/28917/why-did-my-answer-become-community-wiki-history-says-i-did-that-but-i-didnt) and [the CW-FAQ](https://meta.stackexchange.com/questions/11740/what-are-community-wiki-posts-on-stack-overflow/11741#11741):

> ##How does a post become a Community Wiki post?

> There are several ways a question or answer can enter community wiki mode, and most of these ways will occur automatically based on the rules of the system.

> Posts enter community wiki mode when:

> * The body of the post has been edited six (**6**) times by at least four (**4**) different people.
* The post has been edited eight (**8**) times by the original owner.
* The post's author checks the *community wiki* checkbox when composing the question or answer. <sup><sub>Note that this checkbox is only available to users with a reputation of 15 or more.</sub></sup>
* The post 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. <sup><sub>Note that when a question is made CW after answers have already been posted, the existing answers are not normally converted to CW automatically.</sub></sup>
* The question generates more than **30** answers (**15** on Super User). 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.

> <i>* note that the rules outlined above are based on observations; the actual behavior of SO is subject to change without notice, and may well have done so since this was last updated...</i>