Like Carlos said most likely the EMD update.
See - http://mozcast.com/metrics and tab 'EMD Influence'.
Regarding your SEO people, you may not have access to your webmaster tools account but you could put your website domain so we could take a quick look.
It wouldn't take very long to tell if your SEO people are doing a good job or not.
Just so you understand a little better, Google puts some weight (From what I have seen, a lot) on a website which has an EMD (exact match domain) and that also targets that term.
I have seen websites rank with an EMD on the first page with little to no link building.
Google has just turned the power on EMD's down. As you can see from my link.
The reason why the guys above are saying your must have a poor link profile is because even when Google turned the power down, the link building you should have should of held you up.
So... EMD turned down, poor link profile (which should have held you up) = drop
Answer - Build your link profile up.
But it maybe something else until we can see your site, for all I know your whole site is duplicate content and you have been penalised.