Matt Cutts had some time ago said that Google is doing data pushes every day which is resulting in changing of ranks. But some people have asked for more information that influences the ranks. Matt Cutts have reviewed some sites and talks about why the sites loose their ranks and what should be done to get good ranks.
He says “ my general advice would be to take a step back and ask “What value is my site offering to users?” There should be some good reasons that leap to mind. If not, brainstorm why you’re doing the site. Ask yourselves if you took any shortcuts because you were trying to get a good site up quickly. If so, ask if it’s time to revisit any of those decisions and upgrade or augment parts of the site with different tools, features, content, etc. Just to reiterate the point, the issues that I just mentioned are not insurmountable, but if you’re not ranking where you want to be, that’s an opportunity to look for a new approach and other ways to deliver a compelling site to visitors.”
He also makes it clear that Googlebombs are not affecting the ranks but it is rather due to data pushes happening daily.