I'm sure you have a good idea yourself what is going on here, given you have some good experience in online marketing etc. Most likely, one or multiple of the

I'm sure you have a good idea yourself what is going on here, given you have some good experience in online marketing etc. Most likely, one or multiple of the algo's to determine value within results has been modified, which is a regular occurance. Has picked up the content on your site as not really beneficial / trustworthy for whatever the results are and so it has demoted your position. Given almost every page of your site has at least 1 external referal link, its easy enough to imagine it being picked up as an auto generated site, with slapped in affiliates. Infact im surprised it hasnt happened sooner. I'm not sure what you can do about it, other than reworking your entire site, obfuscating the referal links somehow and then hoping for the best. Or branch out into other methods of content and earning if this is the only source of income for you, its rough, but that google algo and others are black boxes that you cant really depend on.. Eventually it will clear everyone off that's not investing in staying on top of these things.