It will be google's new update its mean that google will degrade those website have not good content and on-page but they having good position in SERP due to their SEO.

anything that is not natural will be over optimizing. Tricks that we use to get ranked on the backend of pages....the way around this is by using multiple media on your pages and getting natural backlinks through social media and comments.

Keyword stuffing/density is the most obvious sign of an over optimized web page.

If it looks like spam, it's spam.

Here is a link to a post from SEOmoz explaining it all.

Over [SEO] optimization is exactly as it sounds -- when you focus so much on optimizing for SEO that you're attempting to game the system rather than develop something useful that's reasonably optimized. Engines know you need to do certain things to appear -- like make accessible links, name your pages well, use header tags -- they want and need this. What they don't want is sites that never link out or flag every link as nofollow -- links are useful. They don't want sites that harvest, trade and build up links just for seo "juice". They don't want pages written to keywords and length only.

When you reach a point where everything is about an SEO trick you stop being useful and that's when you get flagged.

Just think for a while if you don't work on the SEO for a particular website what happens, it still gets some visitors and gets indexed but very slowly, it would also get some irrelevant links by people referring them in some offtopic discussion. Now, when you speed up this natural happening a little there is every possibility that it can get a good brand image.
Because good brands get popular quickly and naturally.