Google penalizes websites for taking part in practices that are against its Webmaster guidelines. These penalties can be the consequence of a manual audit or algorithm upgrades, for example, Google Penguin.
Google penalties can bring about the drop of rankings for each page of a site, for a particular keyword or for a particular page. Any drop in rankings carries with it a noteworthy drop in traffic for the site.
To see whether a site has been influenced by a Google penalty, website proprietors can utilize Google Webmaster Tools and additionally break down the timing of their traffic drop with the timing of known Google updates.