I am the owner of WH, which is a webmasters forum. Url is webmastersheart dot com. I was #11 for the keyword "webmaster forum" without the quotes. I was getting decent traffic. All the sudden, one day, my ranking dropped to 100. I checked all over and I still had all my backlinks and didn't change a thing on the forum. I even have vbseo. Anyone have any ideas why?
webmaster forum keyword phrase is kind of competitive. If your forum are not active like your other competitor you will likely be outranked just by one day of active forum on your competitor forum.
Maybe its temporary, sometimes it happens and again comes back after a few days. Also redspace is right, 'webmaster forum' is a very competitive term. You have to work hard to get back the ranking.
Observe for another few days, do not press the panic button just yet, wild swings does happen in Google, and after a few days it will be back to normal. If it does not, you can start worrying then.
How old is the site Youngher then 1 year ... ? then is google dance it should get back in 2 weeks or so
my site has experience massive changes my traffic has gone from several thousand down to a few hundred.