We recently had surge of visits on our site, usually they are between 30-40, we suddenly got about 340 visits with about as many unique visitors. However, the bounce rate shot up to 96%. It was only the one day. After that, visits and bounce rates have been normal.
What can this be?
It is likely that at article or photo on your site went viral. Tons of users from social news sites clicked through to look and then backed out afterwards. Look at "Content" -> "Site Content" to see if one page got unusual traffic on that day. Also look at "Landing Pages" in that section to try to identify what the users were coming to. Also look at "Traffic Sources" -> "Referrals" to try to identify what sites were sending you the traffic.
Another possibility is that a search engine was ranking your site for a keyword that day. Users were clicking through and then backing out because the keyword was not a good match for your site. The search engine realized this and stopped ranking your site for that keyword. If this is the case then you can identify what term and landing page it was under "Traffic Sources" -> "Search Engine Optimization".
Check to see if there are any other strange occurrences in your analytics... I had a similar problem and noticed all of the new traffic was coming from three specific locations.
It basically turned out to be an up-time monitoring tool which triggers the GA tracking script.
