Most webmasters believe that Google punished their site or blog and moved domain away from top positions. In reality Google punish websites only if found not appropriate content, 'black hat' SEO going on, violation of privacy and so on. Even if site links appear on link farms or mirror sites Google do not punish this website unless whole backlink portfolio is a large link farm.

Drop in Google positions appear when website loose backlinks, site redesign, or Google change algorithms to rank websites. In each cases it is only webmaster fault and Google does respond for this results.

1) If website lost backlinks:

In this case website owner should check who link site using backlink checker tools like LinkVendor.

Try to remember if you were using search engine optimization company service in near past. They might created for you only sponsored links for 1-2 month / year from other sites or this SEO company added your url in hundreds sites which belong to them and this backlinks were removed after.

Maybe you was a member of a website which created link exchange program. Sometimes happen that whole group discovered by new algorithm and all links devaluated by Google algorythms.

Did someone offer you hundereds 'quality links' in the past? Check if they removed.

Remember about Pagerank. It could happen if you lost several pr7-8 backlinks you paid / got before.

2) Site redesign. Often redesign does not hurt website, but if you switch from index.html to index.php installing good content software it is another thing. Having hundreds and thousands links you probably know that some sites might link you as .com/index.html. Also, sometimes Google datacenters count regular link as index.html. To show that your site still exist and make Google count links again... you have to use 301 redirect from index.html to index.php. Bit in this case you loose your pagerank until next update.

3) Google always changes algorithms. Your site could be wiped off from top positions if Google increased or decreased value of some types of links. For example you were building your links submitting your site in directories and over 80% of your backlinks coming from hundreds and thousands directories. Google can easily decrease a little value of all specific software based directories because statistic shows that it is easy to spam and be approved in these directories. Sites who used to have a little amount of directory links still stay on top, but all sites who was 100% concentrating on directory submissions are going down. In the other case Google can create algorithm which decrease value of footer links and increase value of links which on top... your site goes down if you was buying links and many sellers gave you cheapest footer links. So, there are millions way Google can play with their algos and by accident they can hit your site right in the heart.

To avoid drops during algorithm changes you have to build different type of links from your site and move to the link building fields you did not tried yet. If you have all types of links, you can make algorithm change work for you and kick away your competitors. To be friendly with algos you need different links like:

1) Directories submissions

2) Article submissions

3) Links from forums

4) Links from blogs (blog posts)

5) Links from link pages

6) Some high pagerank links (pr5+)

7) Links from related sites

8) Reciprocal links

9) Links from social sites

10) Content links

11) Static links

12) Dynamic links

... and many other types of links from different types of websites which build with different software.

Only quality backlinks from different types of websites can help you to avoid drops in SERPS.