I'm curious as to why some answers show up as community wiki even though the question they're in response to is not marked as a community wiki?
The answerer likely checked the community wiki checkbox when composing or editing their answer:
Generally you can find out when a post was made community wiki by viewing its history. For example, this answer was created a community wiki answer, per the first history item at the bottom of the page.
Note that it's also possible for posts to turn into community wikis due to multiple edits, per the criteria listed on the community wiki FAQ entry. This is applicable for both questions and answers.
Since it's at the answerer's discretion whether or not to make their answer community wiki, it's hard to say why people may do it in every case. However, there are various reasons why this option is made available.
Are you asking how or why? Assuming the former, answers can be marked as community wiki even if the question isn't marked as such. In fact, only answers can be marked as CW by the poster; one must ask a moderator to mark a question as CW (both questions and answers can "age" into CW, as Tim Stone describes in his answer).