The internet meme of "creepypasta," probably. See /r/nosleep for the sanitized version. This is the meme cluster that yielded Slenderman, BEKs, and the SCP project.
Well, Slenderman had the video game Slender popularize the meme, so I would expect the trend to be dominated by people who came to the meme via that game rather than the organic growth. From that chart it looks to me like creepy started spiking around the same time creepypasta did. To be fair, creepypasta is the name of a genre at its origin source, whereas most people are going to see stories downstream of the origin and search for them with queries like "creepy black eye kids" or something.
I guess I would qualify that I think this meme is probably what caused the 2007 spike. Later growth in the creepy space is probably more due to online dating becoming popularized; this would allow creeps to be creepy with virtually no possibility of retribution.
What's sanitized about /r/nosleep? I've poked through that sub a little, but I haven't gone specifically searching for creepypasta elsewhere, so I haven't been able to see a difference.
Reddit tends towards sanitization because it's so easy to kill something on /new and so hard to break into /hot on bigger subreddits. On any subreddit where new posts aren't immediately on /hot, only the cadre that browses /new is going to influence what is allowed, and only the things that pass that cadre and are massively palatable are going to become popular.
On /r/nosleep, I think this plays out just as somewhat more tame stories becoming popular. People are going to downvote stories that actually make them uncomfortable. The demographic on reddit is older and so a lot of the stories are family-centric which I think takes away from the horror (there aren't many Lovecraft stories where his adorable 3 year old is actually something else, because Lovecraft wouldn't want to write about an adorable 3 year old in the first place).
To see this in the large, you can check out the top post on /r/nosleep of all time, which is this ama-style bag of short anecdotes told from the point of view of a search and rescue ranger: https://www.reddit.com/r/nosleep/comments/3iex1h/im_a_search...
It's very much in the style of a usual reddit post, making it accessible to redditors. It has many short, shallow anecdotes, which is good for upvotes because if someone likes at least one, they'll upvote it, but it's bad for the horror, because rather than something longer-form that expands on a single theme, you get these little snippets that aren't very immersive. It's creepy, but not horrific.
You'll find they peaked in 2014, and it took them until 2010 to become popular.