Can happen to the best of the sites. It all depends on who they use for ad generation.FYI... I received a malware/trojan horse alert and attempt after clicking this link. Maybe I inadvertently clicked something else on the site. Just wanted to pass it along...
Here is how it works.
Bad guy buys an ad on a provider that allows them to change the ad later. Or directs the content to their own web server. (likely a hijacked one)
Provider validates the ad and starts running it.
Eventually they change the ad to link to hostile content.
And thats how you got there. And since the ad networks dont run the same ad every time to every person, it takes a while for them to figure out because its not constant.