Any Information on Panda Update #21?

[d50-Media]

A site we've been working on and doing well with (relatively new) got hit pretty hard on November 4th and 5th. Since then, traffic is down considerably and we're trying to diagnose what specific triggers may have caused such a change.

[SEO5Team]

Seen a little spike in traffic since the first week of Nov however that can also be attributed to seasonality. We deal with a lot of online stores and charity/ donation portals and this is one busiest times of the year for them. The rankings don't seem to have been impacted that much.

[dmccarthy]

I have I similar issue with a client in the travel industry. They got a panda #21 slap, and I am not sure why. They have quality unique content, good visitor/time on page, no advertising, good load times and sensible use of title and h1 tags.

The interesting thing is that the penalty does not seems to be site wide,their blog on the same domain was not affected, and they are still holding position for main keywords.  However for secondary keywords & long tails they are have dropped and in my opinion are being outranked by sites of lesser quality.

I to having been researching possible causes and am at a bit of a loss...

Anybody else have any input?