Lately I changed one of my site's layout to be responsive. This meant replacing <table> with <div> and moving styles to CSS, etc.
In addition, blocks of HTML, such as the menu and the Ads, may have been moving up or down in the overall code.
The content stayed exactly the same.
I am seeing a decrease in the SERPs, by around 20% - 25% in the pages that I had changed. The decrease was not sudden but over a period of 2 weeks, maybe because crawing of these pages took a few days.
Is it possible/common to loose this amount of ranking due to design changes? Or does the problem lie somewhere else?