We have been on the first page of Google for a number of years for search terms including 'SEO Agency', 'SEO Agency London' etc.
A few months ago we made some changes to the design of the home page (added a blog feed), and made changes to the website sitemap.
Two days ago (two months after last site changes were made) we dropped subsantially in the SERPs for all home page keywords. Where we are found, a date appears before the description in the SERPs, dating February 2012 (which is when we launched the original website). The site has been through a revamp since then, yet it still shows 2012.
This has been followed by a few additional strange things, including the sitelinks that Google is choosing to show (which including author bio pages showing in homepage site links), and googling our brand name no longer brings up sitelinks in the SERPs.
The problem only affects the home page. All other pages are performing as standard.
When Penguin 4.0 came out we saw a noted improvement in our SERP performance, and our backlinks are good and quality, largely from PR efforts. Of course, I would be interested in additional pairs of eyes on the back links to see if anyone thinks that I have missed anything!
We have 3 of our senior SEOs working on trying to figure out what is going on and how to resolve it, but I would be very interested if anyone has any thoughts?