viral - 7:32 am on Feb 14, 2013 (gmt 0)
Linking to relevant sites can help with SEO. Why do you think so many people seem to link wikipedia from their articles?
For some reason this person has picked your site as an authoritative site to link to in their niche.
If they are doing neg seo then it is a really poor attempt. For $5 they could go to fiverr and blast you with 75000 or more spammy comments!