Come on, cut me a break even though I was not 100% precise with my language! I mean browser exploits in general. Unpatched plugins, XSS, spoofing, etc. If they didn't clean up after themselves you could try to correlate an attack with something in the browsing history and try to evaluate it and add it to a shared blacklist. I am not endorsing that, just speculating that could be the well-meaning (if dumb) explanation for this.
I didn't mean to scrutinize - I just know very little about browser exploits, especially details, and thought there was some specific attack for browser history.
