My suspicion is that one of the ad vendors they use has allowed a "nasty" through - not the first time it's been spotted (not with this site though AFAIK). It's a good wheeze on the part of the b***ards - buy some advertising space, let the "agency" vet the ads, then switch them for something with some malicious code in it. Et voila - malicious code served up to users in adverts on multiple websites that the user trusts.
I'd guess that there's some javascript in one of them that's triggering the redirect to another URL, then that redirects, and so on through a few sites till it hits their "drive by download" site. The redirects will be to avoid AV packages spotting the URL in the code and blocking it at source.