I think it's probably part of the Google algo. Maybe something like a ratio of tweets and blog posts compared to links.
I'm just brain-storming here, but maybe Google feels that if a site is getting a lot of links very quickly, that this site should also be getting tweets and blogged about as part of its popularity.
I know Google loves the "blogoshpere" and believe that they use this a lot in their algos. By "blogoshpere", I mean a huge network of TRUSTED blogs.
Get a large sample of sites you trust and use them as a "control" to establish a trusted rate of link growth.
Again, I'm just guessing and even if I'm right, there's no telling exactly how much it impacts the SERPs.
On the other hand, what is "natural" and how does Google attempt to figure that out?
-Boom boom boom boom.