If you did a major overhaul, where you pages hardly resemble what they looked like before, you might check to see if the visitor is a first timer or returning. If She was a returning and bounced it may be that they didn't recognize the new look and feel of your site and bounced to check the search query to make sure they typed in what they thought.
I am also doing a site redesign this week so I will keep an eye on this also and report back my findings to this topic.