I think it may be important to state that the only real "ding" is within your Analytics. Google does not use Analytics data in its search algorithm. So if you are monitoring bounce rate for that purpose, worry not.*
More than likely we're looking at a tracking anomaly of some kind, that, at its very worst, is throwing off your Analytics data.
- I might add that the "bounce rate" that DOES influence the search algorithm isn't really bounce rate, but dwell time. See what Duane Forrester wrote about here: http://www.bing.com/community/Site_Blogs/b/webmaster/archive/2011/08/02/how-to-build-quality-content.aspx
This metric is calculated internally by the search engines, and does not cull from Analytics data. Additionally, it is one of thousands of signals that influence ranking, and likely a relatively small one overall.