Google Instant Previews launched earlier this month, may be skewing your web analytics data, as it has created confusions to Google Analytics and other web analytics tracking.
It appears that in some cases Google will conduct an on-demand fetch of your page to dynamically create an Instant Preview. The on-demand fetch happens when a searcher places his mouse over the search result on Google and the image preview comes up.
Some analytics tools, including Google Analytics, may consider that a visit, because Google Instant Preview is actually visiting the page in real-time to get that on-demand Instant Preview.
There're several complaints about this issue in the Google Help forums. Google's John Mueller replied on fixing this from showing up in Google Analytics: