The number of pages indexed on a UK site I work on has gone from 2,500 to 6,500 over the course of a week/10 days (Google site: operator).
No changes have been made to the site, no new pages have been added, and it is a pretty average, commercial site - nothing spammy and very good domain authority.
No new pages or links have been added - pretty much nothing on or offsite to comment on!
The only thing that has happened in this time is a drop in rankings for some major head search terms, which I believe to have been a result of a website outage which happened in this period and was pulled up in Webmaster Tools.
It looks as if our main competitors have also had massive changes in the number of pages indexed...
Sales are down and we are not ranking for any new tail terms. Does anyone know what might be going on?