Well if they were actually adding something to their cart, that implies they were interacting with the site.
Did you notice anything odd in your log files or analytics? Was all the extra traffic coming from the same source (direct, Google, a website link)? If it was coming from search, did they use the keywords you'd expect them to? Did they all arrive on the same landing page?
Then I'd look and see if they were all using the same browser (maybe there's a cart problem that's specific to one browser) and if they all "fell out" at the same point in the funnel. Did they all look at a number of pages, or only one?
There's a few of the things you want to rule out.