Some time ago Google has released a new algorithm that is "Penguin", if your website traffic decline may can be concluded if you are also affected by this new algorithm.
Common problems is because of keyword stuffing and bad backlinks. For keyword stuffing, sometimes we do not intend to do that, just because of our habit of writing. The most difficult to overcome is if we hit because of a bad backlinks.
There is an interesting comment from Webwork which is a moderator of WebmasterWorld.com:
Google has done more than any other entity to screw up natural linking. From instigating the era of automated guestbook / forum / webstats / blog link spamming (talk about link removal headaches), TO the era of the spamming of email link requests, TO the era of PR notice blasting, TO the era of NOT linking because linking might drain your "link juice", TO the no-follow era, TO the era of not even linking for money because paid links are EVIL, TO the era a link/site-bowling or link attacks, and now the era of unlinking. Stop the madness. This seemingly rational, pragmatic, intelligent, even necessary conversation - about how to deal with what Google says is unnatural links - needs to stop. Unlinking requests, as a business practice or mandated necessity, is simply a bridge too far.
Read the whole tread here.
Microsoft often blamed break the web with their browsers, and Google in the future may often blamed for broke the website owners with their algorithm.