I was somehow affected last Friday and what I see now is a problem with the canonical "www" VS "non www"
If I search for site:mysite.com, the sites that were affected also return a "non www" version and appear in Google further down the field in the "non www" version.
The sites that were not affected are in normal "www" version.
I haven't made any changes so it's somewhat strange to see this behavior since until last week all was OK in terms of canonical. (I have a .htaccess 301 redirect from "non www" to "www"
Maybe some of you can check this out and reply if you notice something.
(Or maybe this is a consequence rather than a reason...)