Panda 4.1 rolled out September 25 2014 according to:
The latest update comes four months after the last, which suggests
that this might be a new quarterly cycle that we’re on. Panda had been
updated on a roughly monthly basis during 2012. In 2013, most of the
year saw no update at all.
Of course, there could have been unannounced releases of Panda that
have happened. The list above is only for those that have been
confirmed by Google.
Some argue that the updated started before the announcement.
Here are some more links:
The up-shot of the update is this:
Panda 4.0 while effective, did not solve the entire problem and some target sites were left standing. These sites were analyzed and Panda was updated to handle the tactics that allowed these sites to survive Panda 4.0.
Based on user (and webmaster!) feedback, we’ve been able to discover a
few more signals to help Panda identify low-quality content more
precisely. This results in a greater diversity of high-quality small-
and medium-sized sites ranking higher, which is nice.
--Pierre Far, Google Webmaster Trends Analyst
If your site fits the criteria for Panda 4.0, it is possible that you got hit. The link http://marketingland.com/panda-4-1-changes-content-performance-strategy-103850 gives clues to what is targeted in this update. This site goes into much more detail: http://www.hmtweb.com/marketing-blog/panda-4-1-analysis/
It is possible that you got hit by Panda 4.1 despite the announcement date. It appears that your Direct (none) closely mirrors your sessions. Not sure what that means exactly, but the timing may be there even though it is a month later.