There are several reasons but three are given below:
1. Webmaster always wants to update in his website and changed the web pages. They have no knowledge about the ranking factor of google. Mostly time it happened that after a web page change, Google temporarily down the ranks of web pages as well as your website. In this situation, you don't have to worry about the ranking drop because you'll get a hold your old as well as better rankings back, but it will take some time.
2. Time to time the web pages of your website rank drop because your web pages don’t change long time and when you see other websites similarly with you site web pages are better than yours web pages. You would be think that your competitors might have built better web pages with better content and better inbound links. So for better web pages change the content at regular interval and optimize the web pages with better keywords and try to get better inbound links.
3. The Webmaster always finds, Google thinks that if you use spam elements on your web pages, in response he penalize to your website. So keep away from spam in your web page’s content.
Above all things take part in Google ranking algorithm.