First bounce rate, as I know it to be, is when a visitor abandons the site before sitting on it long enough for it to count as a visitor. I have noticed from time to time that I got a huge spike of Google visitors hitting my site (when I did an Arin's, they resolved as Google). As a result, it caused my bounce rates to be very high.
Now, if legit traffic is jumping from your site it is either because your site is taking way to long to load or that you don't have anything worthwhile on your home page to keep them there or direct them to a subpage.
So look carefully at your analytics at the IP addresses and see who is hitting it and what pages. Look at the frequency.