Google updates it's algorithm with 'Penguin' this month which targets webspam (black hat) SEO. Google always keeps information about it's updates 'close to its chest' but usually it is a good idea to remind ourselves about Googles quality guidelines. Googles Matt Cutts said "our advice for webmasters is to focus on creating high quality sites that create a good user experience and employ white hat SEO methods instead of engaging in aggressive webspam tactics" in the Penguin announcement 

He also said, “The change will decrease rankings for sites that we believe are violating Google’s existing quality guidelines.”

So, its about quality guideline which includes hidden text and links, cloaking, keyword stuffing, links...etc. If you need to check with your SEO team and make sure you are clean up your any webspam tactics that might have been used to push you up the rankings before your lose your traffic.