claaarky - 9:11 am on Mar 2, 2013 (gmt 0)

If a site loses traffic for the first time at a Panda update it's possibly due to a change in Panda which now affects you. If it happens at a Panda refresh then it's a signal, as Panthro says, your competition has raised its' game and your site is no longer viewed as high quality relative to everyone else.

If your traffic had been going up significantly prior to the Panda slap (overall or for one particular part of the site) it could be that a quality issue has been exposed and the whole site is now demoted.