This suggested edit was declined by the user who created the tag wiki excerpt, who doesn't yet have the reputation to be a trusted user. (The site is still in beta, and the reputation necessary to be a trusted user is 4000.)
Should the user who first proposed a tag wiki, or a tag wiki excerpt be the one who can unilaterally decide about a proposed edit for a tag wiki?
The user who first proposed it isn't necessarily the one who knows if the tag's purpose can be better described with different words, nor does that user necessarily know if the tag has been lately used differently from how it was first used. Since a tag wiki is different from other posts, there should be an exception to the rule "the author of a post can unilaterally decide about any edits."
The paradox is then that the creator of a tag wiki still needs two users to approve their suggested edit. That's what happened in this case.