Urk. Does it do it all the time, or only after you've visited certain pages? The back-button hijack via site code is old; a cursory lookup turns up articles going back several years. (Oldest I saw was 2009-- there's even a post or two here [webmasterworld.com]-- but needless to say I didn't look very carefully.)
If they've found a way to commandeer your back button full-time, this is something new and it's bad news. Er, worse news.