jmccormac - 6:01 am on Aug 23, 2013 (gmt 0)
As Robert Charlton said above, there are some rather nasty injection hacks that only appear if you use a non-standard user agent. The site might appear to be clean otherwise. Do a Google search for your site (site:) with common drugs/pron/warez keywords to see if this is the problem. These hacks tend to occur at a control panel level or because of an outdated Joomla/Wordpress plugin issue.
One of the major problems in the last year has been that Wordpress plugin compromise that allowed spam links to be injected. Google, in its finite wisdom, still hasn't managed to deal with this problem as it applies to compromised sites and many compromised sites are still in its index.
The other issue is that domains drop and they are sometimes picked up for their link characteristics. If your site has been going for a few years then there is a high probability that some links that were previously good have gone bad or are pointing to PPC parked webpages. What you may have is an accumulation of dropped/reregistered domains.