The answer to the former turns out to be a setting in Google Chrome called "Use a predication service to load pages more quickly" (It used to be called "Prefetch resources to load pages more quickly" in earlier builds of Chrome). Interestingly this is categorized as a "Privacy" rather than a "Performance" setting. What this appears to do is wait for the initial search results, then automatically download the topmost website in the search, presumably on the assumption that this is the one you are most likely to click on next.
In our scenario the assets include a virus and as soon as this is downloaded ESET AV inspects it and, judging it lacking in moral fiber, it quite correctly
stomps it's baleful binary guts out.
For the latter question of whether this represents a real threat, it appears that the answer is "no". If all Chrome is doing is caching web site content that might be needed in the near future then the malicious content cannot be executed at this stage even if the local AV client fails to destroy it.