When Google changes its algorithm?

Well, usually it happens each quarter. Although, Google folks would tell you that Google changes its algorithm constantly, without a patern, the major ups and downs happen about 4 times per year.

How do you know about them?

• Well, you suddenly see a change in:
  
  • your Google PR rank
  • number of indexed pages per site
  • number of backlinks to your site, captured by Google

• You'll notice a buzz around this issue around (LinkedIn, Yahoo Questions, Xing, eCademy, forums, etc)

• You'll check Google's most popular blogs, such as:
  
  • http://googleblog.blogspot.com/
  • http://analytics.blogspot.com/
  • http://www.mattcutts.com/blog/
  • and so on...

• You'll search in Google: "Google algorithm change" and get lot's of valuable information regarding this point

 Why does a change in Google algorithm results in drop-down of indexed pages? There are many reasons for that. Let me list a couple:

• Google revisited your site and finally realized that some web pages do not exist anymore - and removed them from Google 'cache'
• Google revisited your site while it has been down or you've exceeded allowed bandwidth

 Why did a change in Google algorithm resulted in change of your Page Rank? The list is huge, but let me mention several things:

• Google re-evaluated the backlinks pointing to your site in terms of relevancy, age of the domain where your site has been mentioned, number of back links to that site, where your link has been mentioned, average link grade, average number of links per month, and so on...
• Popularity of the site changed naturally, or Google noticed some unusual spikes and re-evaluated them
• Site needed a better and up-to-date on and off-site optimization (tune-up)
• Site had been over-advertised
• And more...

These answers, of course, do not cover your specific problem, but may give you a clue.

I'll be happy to help with more detailed site analysis.