WWW Version Cache Deleted

A 5 year old site. non WWW vision still OK.

[GuluGulu]

My site www. domain.com has a PR of 5 and non WWW vision has 4. Both were showing in the SERPS and cached regularly. 3 days back I have seen the Www version of home page is removed from cache and PR dropped to 4. If was in cache since last 5 years. I have made some HTML changes to my site, that has around 1800 pages in total.

After I updated my site, G deleted the cache for WWW version home page and cache is updated with the updated home page for non-WWW version. Interestingly, G has cached all internal pages that has similar HTML update and that cache is showing now. If is wonder and I can not figure out what has happened at G.

The same WWW version in proudly showing at the for first page SERP of Yahoo and MSN with right updated cache. But G has deleted it.

Can anybody please explain?

I have still same traffic from all SE.