Okay, I'm going to try to word this as clearly as I can. I'm using the 3 websites as examples because when I just used numbers it seemed confusing. This issue happens with two or more tabs.
When I have a few tabs open to sites like facebook and an mmorpg, THEN I open another tab for say IMDB to look up something, as the IMDB tab loads the facbook tab will also become an IMDB tab. (2 tabs to the same site)
It doesn't happen all the time, but I have noticed that it happens more often if I leave an action open on another tab, like I start writing something on facebook then go google something before I finish, the facebook page will change to the google page.