Mmm... yeah hard to guess without looking at the site then, on my own experience / research, these are some of the issues I found in many of the sites affected by Panda:
•Intrusive advertising, excessive use of Adsense, sites created only for Adsense or to solely promote a product•High amounts of duplicate content / scraped content•Bad user interface / “ugly” design•Usage data - low click-through-rate, low time-on-site, 100% bounce rate•Content analysis - not usable/readable/easily-consumable content•Excessive internal linking to one or two pages only
And I don't mean your site to be spammy, but some cases, like news sites with advertising, sometimes they get articles out with just a couple paragraphs of content, so that single page becomes more advertising than content.
Consider posting your site, it would be nice to take a look
and there is also the last reason: your site is innocent and just got hit by mistake, it happens.