I have a site and everything went well in the past.

But for some reason I made some changes to the site and after that I noticed more and more indexed pages went to Supplemental Result in Google. I am just wondering why this has happened and how I should do to avoid this.

Thanks for your input.

unique titles and meta descriptions often helps as does better quality backlinks

What are supplemental Results?

The supplemental results are "orphaned pages", or pages that would normally be dropped from the index because they don't show any (or a minimum number of) backlinks.

or watch matt cutts video about Supplemental Results
http://video.google.com/videoplay?do...13828170903728

There are a lot of theories about what is causing this. It's not just a matter of lack of backlinks, although that is one of many theories. Probably 95%+ of all web pages have no backlinks and the overwhelming number are not going supplemental. I've seen duplicate content, lack of content, lack of outbound links, old content, orphaned pages and a whole rash of issues that could possibly lead to pages getting trashed into Supplemental Results. I've also seen sites with 100% unique content, lots of inbound links, unique title and meta tags and absolutely no spam get tossed into supplemental.

I'm currently testing the unique titles and meta tags theory on a couple of web sites that were tossed into Supplemental Reults without any other apparent reason. I've seen some compelling evidence that this does work over time.

Matt really downplays the significant negative impact that Supplemental Results has on a site's traffic and income. He doesn't really say anything meanful in that video; he just addresses the question of what it is in vague terms.

I did have one site pull out of supplemental results about a week after filing a reinclusion request that pointed out that the site is in full compliance with Google's guidelines. Personally, I think their algorithm screws up sometimes and accidentally trashes a site. In those cases, it may take an engineer's eyeballs to confirm this and take the supplemental flag off of the site.