I've two sites. The first one had been stable for 10 months. It's a govt job site. But suddenly one day( in Feb/2014) the traffic fell by 90%. At the same time I observe it's Index Status graph tumble down.
Here is the graph
Similar story with my another site which was new but went ok for 5-6 months.
I've never worked to build even single back link or even optimize the site. Here are the GA stats for the first site.:
Bounce Rate: 50%
Avg Session Duration: 3:46
So Google Index falling in the GWT should be proportional to fall in rankings.
I can guess that Google index falling could be due to duplicate content, thin content etc. But it took so many months to realize it!
Another theory I've is that it is the Google's Panda( now built-in ) which has drowned my sites.