OMZen - 7:25 pm on Mar 2, 2011 (gmt 0)

That image does not tell full story.

In your GA, look for "Intelligence"(beta ) and check the Custom Alerts. You should see a long bar on the days your traffic spiked up.

Click on it and scroll down a bit to see what was the source / medium that caused the spike.

Also, don't look at the bounce rate in isolation. Apply second dimension to gain insights into who is sending crappy traffic.

(P.s: on second ( random ) thoughts, makes me wonder whether you experienced this around the time farmer update rolled out.. )